ERRORS   Ol 

ACCOMMODATION 
AND  REFRACTION 


CLARKE 


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BttKEllY 

LIBRARY 

UHivERSfTY  or 

CALIFOftNIA 


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THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 


THE   ERRORS  OF  ACCOMMODATION   AND 
REFRACTION  OF  THE  EYE 


THE    ERRORS    OF 

ACCOMMODATION    AND 
REFRACTION  OF  THE  EYE 

AND    THEIR   TREATMENT 
A     HANDBOOK     FOR    STUDENTS 


BY 

ERNEST   CLARKE,    M.D.,    F.R  C.S. 

OPHTHALMIC   SURGEON    TO   THE    KING   GEORGE    HOSPITAL,    QUEBN    ALEXANDRA 

HOSPITAL    FOR   OFFICERS,    ETC. 

CONSULTING    SURGEON    TO   THE    CENTRAL    LONDON    OPHTHALMIC    HOSPITAL 

CONSULTING   OPHTHALMIC    SURGEON   TO    THE    MILLER   GENERAL    HOSPITAL 


FOURTH    EDITION 


NEW  YORK 
WILLIAM     WOOD     &     COMPANY 

MDCCCCXVIII 


OPTOMETRY 


Printed  in  Great  Britain 


Sb--#       V**" 


orro 

PREFACE  TO  THE  FOURTH  EDITION 

The  first  edition  of  this  work  was  published  fifteen  years 
ago  based  on  lectures  delivered  at  the  Central  London 
Ophthalmic  Hospital  and  the  Medical  Graduates'  College . 
I  have  not  altered  its  character,  which  is  essentially 
practical,  all  matter  unnecessary  for  the  busy  practi- 
tioner or  overburdened  student  being  omitted. 

The  whole  work  has  been  thoroughly  revised  and 
brought  up  to  date,  many  chapters  having  been  re- 
written. 

The  subject  of  Eyestrain  still  occupies  the  prominent 
place  that  is  its  due. 


ERNEST  CLARKE. 


Chandos  Street, 

Cavendish  Square,  W. 
February,  191 8. 


CONTENTS 


CHAPTER  PAGE 

I.    REFRACTION PRISMS LENSES                  -                  -  -             I 

II.    OPTICAL  PROPERTIES  OF  THE  NORMAL  EYE         -  "19 

III.  ACCOMMODATION              -                 -                  -                 -  -         29 

IV.  CONVERGENCE                     -                  -                  -                  -  "39 
V.    THE  OPHTHALMOSCOPE                    -                  -                  -  "57 

VI.    HYPEROPIA         -                  -                  -                  -                  -  -          85 

VII.    MYOPIA                  -                  -                  -                  -                  -  -         99 

VIII.    ASTIGMATISM     -                   -                  -                  -                  -  "115 

IX.    PRESBYOPIA       -                   -                  -                  -                  -  -142 

X.    ANISOMETROPIA                 _                  -                  -                  -  -       152 

XI.    APHAKIA  _-----       159 

XII.    EYESTRAIN        -                 -                 -                 -                 -  -1 62 

XIII.    HETEROPHORIA                   -                  -                  -                  "  "I?! 
XrV.    STRABISMUS      ------       184 

XV.    CYCLOPLEGIA,  CYCLOPLEGICS,  AND  CILIARY  SPASM  -       I98 

XVI.    METHODS  OF  EXAMINATION NOTE-TAKING        -  -       202 

XVII.    SPECTACLES       ------       208 

XVIII.    ILLUSTRATIVE  CASES     -                  -                  -                  -  -2I3 

XIX.    VISION  TESTS  FOR  THE  SERVICES              _                  -  -       221 

BIBLIOGRAPHY  -  -  -  -  -      23O 

INDEX  ------       232 


VU 


ERRORS    OF    ACCOMMODATION 


AND 


REFRACTION   OF  THE   EYE 

CHAPTER  I 

REFRACTION— PRISMS— LENSES 

Light  is  propagated  in  straight  lines  which  diverge 
from  any  luminous  point  in  all  directions,  and  these  lines 
of  direction  are  called  "  luminous  rays."  The  propaga- 
tion is  produced  by  ether  waves  which  are  across  the 
path  of  light.  The  velocity  of  light  is,  in  round  numbers, 
186,000  miles  per  second,  and  is  appreciably  retarded 
in  passing  through  a  denser  medium.  Rays  of  light 
entering  the  eye  coming  from  any  luminous  point  at  a 
greater  distance  than  6  metres  may  be  assumed,  for  all 
practical  purposes,  to  be  parallel. 

Light  is  absorbed,  refracted,  or  reflected. 

Refraction  of  Light. — A  ray  of  light  passing  from  a 
rarer  into  a  denser  transparent  medium,  if  it  be  perpen- 
dicular to  the  surface,  and  the  boundaries  of  the  medium 
be  parallel,  will  pass  out  of  the  denser  medium  in  the 
same  straight  line  (Fig.  i,  l),  the  only  effect  upon  it 
being  a  retardation.  If  the  ray  enter  the  denser  medium 
other  than  perpendicularly,  or  if  the  boundaries  of  the 
medium  be  not  parallel,  the  ray  is  bent  or  refracted. 

A  simple  illustration  will  explain  this. 

Explanation  of  Refraction. — i\s  ether  waves  are  at 
right  angles  to  the  path  of  light,  if  a  beam  of  light  enter 


2  TH£   RtFRACTION   OF  TH£  EYE 

a  denser  medium  obliquely,  one  end  of  the  wave  will 
enter  the  denser  meditim  before  the  other,  and  conse- 
quently be  retarded  earlier.  Let  A,  B,  c,  d  (Fig.  i),  be  a 
denser  medium,  with  parallel  boundaries,  and  n,  o,  p,  q, 
the  beam  of  light. 

The  wave  front  will  reach  Q  before  it  reaches  R,  and 
it  will  at  once  be  retarded ;  and  as  it  thus  travels  more 
slowly  from  Q  to  s  than  from  p  to  R  (which  is  outside  the 
denser  medium),  the  beam  must  be  swung  round  so  that 
it  is  bent  or  refracted  on  entering  the  denser  medium. 


Fig.  I 


Across  the  denser  medium  the  whole  wave  front  is 
equally  affected,  so  that  the  beam  passes  across  in  a 
straight  line ;  and  if  the  sides  of  the  medium  be  parallel 
the  converse  happens,  and  it  is  again  bent  on  passing 
out,  the  incident  and  the  emergent  rays  being  parallel. 

Let  this  be  applied  to  the  case  of,  for  example,  a  prism 
where  the  sides  of  the  denser  medium  are  not  parallel. 
Suppose  A,  B,  c  (Fig.  2),  to  be  a  triangular  strip  of  velvet 
pasted  on  a  smooth  board,  and  suppose  d  to  be  two 
small  wheels  connected  by  an  axle  in  such  a  way  that 
each  wheel  can  turn  independently  of  the  other.     Roll 


REFRACTION  3 

the  wheels  up  to  the  velvet  triangle;  the  lower  or  right 
wheel  will  pass  on  to  the  velvet  at  e  before  the  left  wheel 
reaches  it,  and  as  the  velvet  will  retard  its  progress,  it 
will  turn  now  more  slowly  than  the  left  wheel,  so  that  the 
pair  of  wheels  will  be  slewed  round  towards  the  base  of 
the  triangle.  When  the  left  wheel  enters  on  the  velvet  at 
/,  its  progress  will  be  the  same  as  that  of  the  other  wheel, 
and  the  pair  of  wheels  will  cross  the  velvet  now  in  a 
straight  line ;  when  it  reaches  w,  the  left  or  upper  wheel 
will  leave  the  velvet  earlier,  and  will  consequently  travel 
more  rapidly,  and  will  again  swing  the  pair  round,  so 


*^ '/  T----:::7 


Fig.  2. 


that  in  the  transit  across  the  triangle  the  pair  of  wheels 
have  been  bent  towards  the  base. 

It  is  in  this  manner  that  light  behaves;  in  passing 
through  a  prism,  it  is  bent  or  refracted  towards  the 
base.  *W| 

A  ray  of  light  passing  obliquely  from  a  less  dense 
into  a  denser  transparent  medium  is  refracted  or  bent 
towards  the  perpendicular,  and  when  passing  from  a 
dense  to  a  less  dense  medium  is  refracted  away  from  the 
perpendicular. 

Index  of  Refraction. — The  index  of  refraction  of  a 
transparent  substance  is  the  number  that  denotes  the 


4  THE   REFRACTION   OF  THE   EYE 

refractive  power  of  such  substance  compared  with  air, 
which  is  taken  as  the  unit  i.  In  Fig.  3,  let  A  c  be  the 
incident  ray  meeting  the  horizontal  surface  of  water  at 
c,  and  forming  with  p  p',  the  perpendicular,  an  angle 
A  c  P ;  and  let  c  b  be  the  refracted  ray  in  water  bent 
towards  the  perpendicular,  and  forming  the  angle  B  c  p'. 


v^ 

/O 

~~""'^*^^ 

c 

W 

^ 

k 

Fig.  3. 

The  sine  l  m  is  to  the  sine  n  o  as  4  to  3,  expressed 
as  I,  or  1*33,  and  this  is  the  index  of  refraction  of 
water. 

The  following  are  a  few  of  the  indices  of  refraction 
useful  to  the  ophthalmologist : 


Air  . . 

I'O 

Water 

1*33 

Cornea 

1*33 

Aqueous  humour 

1-3379 

Vitreous  humour 

1-3379 

Crystalline  lens 

1-4 

Crown  glass 

1-5 

PRISMS  5 

Prisms. — An  ophthalmic  prism  is  a  wedge-shaped  piece 
of  glass  having  two  of  its  sides,  or  plane  surfaces,  inter- 
secting each  other  at  the  apex,  and  separated  at  the  base, 
which  is  the  thickest  part  of  the  prism. 

We  have  already  seen  that  a  ray  of  light  entering  one 
of  the  sides  of  a  prism  is  refracted  or  bent  towards  the 
base,  and  the  amount  of  this  refraction  depends  upon — 

I.  The  strength  of  the  prism. 

|2.  The  refractive  index  of  the  prism  substance. 

3.  The  position  at  which  the  light  enters  the  prism. 

The  Strength  of  the  Prism  —  The  Numbering  of 
Prisms. — ^The  power  of  a  prism  to  deflect  or  refract  light 
depends  on  the  size  of  the  angle  at  the  apex  formed  by 


Fig. 


the  two  plane  surfaces.  This  is  called  the  refracting 
angle,  and  is  written  with  the  sign  of  a  degree  after  the 
numeral — thus :  4° — which  is  scratched  on  the  surface  of 
the  glass.  This  is  the  old,  and  even  up  to  the  present 
very  general,  method  of  numbering  prisms. 

Maddox  has  suggested  the  word  "  prismetry  "  to  denote  the 
numbering  by  the  deviating  angle. 

IThe  Deviating  Angle  of  a  Prism. — In  Fig.  4,  if  a  ray,  p,  enter 
the  prism,  instead  of  passing  out  at  p',  it  is  refracted  towards 


O  THE  REFRACTION   OF  THE   EYE 

the  base  b  c,  and  away  from  the  angle  b  a  c,  and  is  again  bent 
towards  the  base  on  passing  out,  and  emerges  in  the  direction  / 
(see  page  3).  The  angle  p'  o  f,  made  by  the  backward  prolonga- 
tion of  /  and  the  forward  prolongation  of  p,  is  the  angle  of  devia- 
tion, and  it  is  equal  to  about  half  the  angle  of  refraction. 

To  indicate  that  the  angle  of  deviation  is  implied,  a  small  d 
is  added;  thus,  prism  4°  is  approximately  equal  to  prism  2°  d. 

Angle  of  Refraction.  Angl*  of  Deviation. 

1°  32' 

5°  2°  42' 

10°  5°  26' 

As  the  metrical  system  is  now  universally  adopted  in  ophthal- 
mology. Prentice  has  suggested  the  numbering  of  prisms  on  the 
metrical  plan,  and  the  prism  dioptre  is  the  unit,  designated  by 
the  sign  A  after  the  numeral. 

A  prism  of  the  strength  of  i  P.D.  (i  A)  is  a  prism  that,  at  a 
distance  of  i  metre,  apparently  displaces  an  object  i  centi- 
metre. In  Fig.  5,  E,  being  the  observer,  sees  o  at  o'  apparently 
displaced*  i  centimetre,  the  distance  between  o  and  the  prism 


Fig. 


b?ing  I  metre,  and  the  prism  i  A ;  that  is,  the  apparent  displace- 
ment of  an  object  looked  at  through  a  prism  is  i  per  cent,  of  the 
distance  of  the  prism  from  the  object,  multiplied  by  the  prism 
dioptre. 

A  prism  i  A  apparently  displaces  an  object  3  metres  off,  3  centi- 
metres, and  a  prism  3  A  displaces  an  object  2  metres  off,  6  centi- 
metres, and  so  on. 

Dennett  has  suggested  the  centrad  as  the  unit  for  numbering 
prisms.  The  centrad  is  the  hundredth  part  of  a  radian,  a  radian 
being  the  angle  subtended  at  the  centre  of  a  circle  by  an  arc, 
which  is  equal  in  length  to  the  radius.  A  prism  i  centrad, 
designated  i  v.  deviates  a  ray  of  light  one-hundredth  part  of  the 
arc  of  the  radian. 

It  is  interesting  to  note  that  the  centrad  has  a  relative  value 
to  the  metre  angle,  in  that  half  the  number  of  centimetres  between 
the  pupils  indicates  the  number  of  centrads  in  the  metre  angle. 

This  method  is  not  much  used,  although  probably  the  most 
scientific. 

*  Note  that  the  apparent  displacement  of  an  object  viewed 
through  a  prism  is  always  towards  the  apex. 


PRISMS  7 

All  three  methods  of  numbering  prisms  are  practically  iden- 
tical for  weak  prisms,  and,  as  it  is  only  weak  prisms  that  an  oph- 
thalmologist can  use,  it  matters  little  what  method  he  adopts. 

Table  showing  the  Equivalence  of  Centrads,  Prism 
Dioptres,  and  Refracting  Angle,  of  the  Six  Weakest 
Prisms.     (Index  of  Refraction,  1*54.) 

Centrad.  Prism  Dioptre.  Refracting  Angle. 

1  I  I* 

2  2'OOOI  2*12° 

3  3*0013  3*i8° 

4  4*0028  4*23° 

5  5-0045  5-28° 

6  6*0063  6*32° 

The  minimum  of  deviation  occurs  when  the  incident 
ray  crosses  the  prism  parallel  to  its  base;  but  in  thin 
prisms — and  it  is  thin  prisms  only  that  the  ophthalmolo- 
gist uses — this  has  no  practical  importance.  Hence  we 
can  neglect  the  position  of  the  incident  ray. 

The  Uses  of  Prisms.— 

1.  To  remove  diplopia. 

2.  To   ease   the  muscles,   and   so  prevent  muscle 

strain  and  subsequent  diplopia  (see  page  176) . 

3.  To  exercise  weak  muscles  (see  page  182). 

4.  To   test   the   strength    of   the   external  ocular 

muscles. 

5.  To  detect  malingerers  (see  page  183). 

In  trial  cases  the  prisms  are  usually  cut  circular,  so 
that  they  can  be  used  in  a  trial  frame ;  the  exact  position 
of  the  base  of  the  prism  is  usually  marked  by  a  line  on 
the  glass  at  right  angles  to  the  base. 

Rotating  Prisms. — If  two  prisms  of  equal  strength  be 
placed  in  apposition  in  such  a  manner  that  the  base  of 
the  one  is  in  contact  with  the  apex  of  the  other,  they 
neutralize  each  other,  and  if  we  rotate  them  in  opposite 
directions  we  obtain  the  effect  of  an  increasingly  strong 
prism. 

Risley's  Rotary  Prism  (Fig.  6)  is  made  on  this  principle. 
If  we  place  it  in  one  side  of  a  trial  frame,  both  eyes  being 
used,  start  from  zero  and  gradually  turn  the  button,  we 


8 


THE  REFRACTION  OF  THE  EYE 


can  ascertain  the  strongest  prism  the  eyes  can  stand 
without  having  diplopia,  or,  if  we  are  deaUng  with  a 
case  of  diplopia,  we  can  ascertain  the  weakest  prism  that 
will  procure  "  fusion  "  vision. 


Fig.  6. 


The  numbers  on  the  frame  indicate  the  refracting  angle 
in  degrees.  The  instrument  can  give  a  total  prismatic 
power  of  30°. 

i  fisms  form  no  images  and  have  no  foci. 


Fig.  7. 

Lenses. — If  two  prisms  are  placed  with  their  bases  in 
contact,  we  have  roughly  a  bi-convex  lens  (Fig.  7,  a), 
and  rays  of  light  passing  through  it  are  bent  towards 
the  base  of  the  prisms — i.e.,  the  centre  of  the  lens;  in 
other  words,  they  converge.     If  the  prisms  have  their 


LENSES 


apices  in  contact,  we  have  a  bi-concave  lens  (Fig.  7,  b), 
and  the  rays]are]bent  towards  the  bases — i.e.,  outwards — 
and  diverge. 

Spherical  Lenses. — Besides  the  bi-convex  (Fig.  7,  a) 
and  bi-concave  lenses  (b),  there  are  plano-convex  (c), 


Fig. 


plano-concave  (d),  converging  concavo-convex  or  con- 
verging meniscus  (e),  and  diverging  concavo-convex  or 
diverging  meniscus  (f).  Rays  of  light  passing  obliquely 
through  any  of  these  forms  of  lenses  are  refracted  or 
bent  towards  the  thickest  part  of  the  lens. 


Fig.  9. 

The  principal  axis  is  a  line  drawn  through  the  optical 
centre  at  right  angles  to  the  lens  (Fig.  8,  A  o),  and  rays 
passing  through  this  are  not  refracted;  all  other  lines 
passing  through  the  optical  centre  not  at  right  angles  to 
the  lens  are  called  "  secondary  axes  "  (Fig.  9,  a  '«). 

Rays  passing  along  the  secondary  axes  are  refracted. 


10 


THE   REFRACTION   OF  THE   EYE 


but  as  the  emergent  and  the  incident  rays  are  in  the 
same  direction,  and  the  refraction  in  low-power  lenses 
is  very  slight,  the  refraction  can  be  ignored,  and  the  rays 
assumed  to  pass  along  in  a  straight  line. 

Convex  Lenses. — Parallel  rays  passing  through  a  con- 
vex lens  unite  on  the  opposite  side  of  the  lens  at  a  point 
called  the  "  principal  focus  "  (Fig.  8,  p  f). 

At  the  principal  focus  an  inverted  real  image  of  the 
object  is  formed.  Let  a  b  (Fig.  9)  be  an  object  at  some 
considerable  distance  from  the  lens.  Any  ray  passing 
from  the  point  a  through  the  optical  centre  of  the  lens  c 
will  be  unrefracted  {vide  supra),  and  the  image  of  a  will 
be  somewhere  on  this  line  on  the  other  side  of  the  lens 


Fig.  10. 


— let  it  be  at  ^;  all  other  rays  passing  from  A  will  be 
refracted  on  passing  through  the  lens,  and  will  focus  at  a. 
In  the  same  manner  an  image  of  B  is  formed  at  b,  and  all 
other  points  between  A  and  B  will  form  an  image  between 
a  and  b,  so  that  we  get  an  inverted  image  a  b  oi  a  b 
formed  at  the  principal  focus  of  the  lens. 

The  distance  between  the  principal  focus  and  the  optical 
centre  is  called  the  "  principal  focal  distance  " ;  it  is  posi- 
tive, and  convex  lenses  are  known  by  the  plus  sign :   + . 

Rays  passing  from  the  principal  focus  (p  f)  through  the 
lens  emerge  as  parallel  rays  on  the  opposite  side  (Fig.  8). 

Divergent  rays  from  a  point  l  (Fig.  10)  beyond  the 
principal  focus  f  meet  at  a  point  /  beyond  the  principal 


LENSES  II 

focus  f'  on  the  other  side  of  the  lens.  If  the  point  L  is 
twice  the  focal  distance  of  the  lens,  then  /  will  be  at  the 
same  distance  on  the  other  side.  These  two  points  are 
called  "  conjugate  foci,"  and  are  interchangeable;  that  is, 
the  object  may  be  at  l  or  /,  and  the  image  is  respectively 
at  /  or  L. 

If  a  luminous  point  be  between  the  convex  lens  and 
the  principal  focus,  the  rays  will  still  be  divergent  when 
they  leave  the  lens  on  the  opposite  side,  and  consequently 
no  real  image  is  formed;  but  a  magnified  virtual  image 
is  formed  beyond  the  principal  focus  on  the  same  side,  at 
a  point  called  the  "  virtual  focus,"  and  this  virtual  image 


Fig.  II. 

is  seen  by  an  observer  on  the  opposite  side  of  the  lens, 
the  light  from  the  points  a  and  h  appearing  to  thr^ 
observer  to  come  from  a'  and  h'  (Fig.  ii). 

Concave  Lenses. — Parallel  rays  passing  through  a  con- 
cave lens  diverge,  and  consequently  never  come  to  a 
focus ;  but  these  divergent  rays,  if  prolonged  backwards, 
will  meet  at  a  point  f  (Fig.  12). 

This  point  is  the  (virtual)  principal  focus  of  a  concave 
lens. 

If  an  object  be  placed  beyond  the  principal  focus  of  a 
concave  lens,  an  observer  on  the  opposite  side  of  the  lens 
will  see  a  virtual,  erect,  smaller  image  on  the  same  side 
as  the  object;  thus,  rays  from  a  and  h  will  appear  to 


12  THE   REFRACTION    OF  THE   EYE 

come  from  a'  and  h' ,  and  the  object  «  6  is  seen  as  a'  h' 
(Fig.  13).  As  concave  lenses  have  a  negative  focal 
distance,  they  are  denoted  by  the  minus  sign:]-. 

Cylindrical  Lenses.— In  addition  to  spherical  lenses 
cyhndrical  lenses  are  required — these  are  lenses  cut  out 


Fig.  12. 


of  a  cylinder;  convex  cylinders  are  cut  from  a  solid 
cylinder  (Fig.  14,  a),  concave  cylinders  from  a  hollow 
cylinder  (Fig.  14,  h),  which  may  be  regarded  as  the  mould 
of  convex  cylinders.     Cylinders  have  the  property  of  not 


I 


Fig.  13. 

refracting  any  rays  that  pass  along  their  axis,  but  rays 
passing  at  right  angles  to  the  axis  undergo  the  maximum 
refraction  corresponding  to  the  strength  of  the  lens. 
According  to  the  angle  at  which  the  rays  impinge  upon 
the  lens,  they  undergo  more  or  less  refraction,  as  this 


LENSES 


13 


angle  is  further  away  from,  or  nearer  to,  the  axis  of  the 
cyUnder.  A  cylinder  has  no  one  focal  point,  but  a  line  of 
foci  parallel  to  its  axis. 

Cylindrical  lenses  are  employed  to  correct  regular 
astigmatism. 

The  axis  of  a  cylindrical  test  lens  is  marked  by  a  small 
line  on  the  glass,  or  by  making  the  sides  of  the  lens, 
parallel  to  the  axis,  opaque. 


Fig.   14. 


Numeration  of  Lenses. — The  lens  whose  focal  distance 
is  I  metre  is  taken  as  a  unit,  and  its  refractive  power 
is  called  one  Dioptry  or  Dioptre  ("  d  ").  A  lens  of 
twice  the  power  of  this — viz.,  2  d — has  a  focal  distance 
of  ^-;  i.e.,  50  cms.;  a  lens  of  half  the  power — viz., 
•5  D — has  a  focal  length  of  2  metres,  and  so  on.     The 

focal  distance  of  a  lens  no  = ' 

n 

Under  the  old  system,  a  lens  whose  focal  distance  was 
I  inch  was  taken  as  the  unit,  and  a  lens  whose  focal 
length  was  10  inches  was  called  jV*  3^  inches  ^,  and 
so  on.  The  great  disadvantage  of  this  method  of 
numeration  was  the  inability  to  make  it  international, 
because  the  inch  is  not  an  international  measure. 

To  convert  the  old  numeration  into  the  new,  divide  the 
denominator  into  40;  thus,  lens  \  is  V-==S  d,  and  vice 


14 


THE  REFRACTION  OF  THE  EYE 


versa,  Jo  convert  dioptres  into  inches,  divide  the  dioptre 
into  40,  and  the  result  is  the  focal  length  in  inches;  thus, 
4  D  =  -*3^=  10  inches  focal  length,  expressed  as  to- 

The  following  table  shows  at  a  glance  the  approximate  equiva- 
lent of  the  old  and  new  numeration : 


)ioptres. 

Inches. 

Dioptres. 

Inches. 

•12 

320 

4 

10 

0-25 

160 

4-50 

9 

0.37 

107 

5 

8 

0-50 

80 

5-50 

7 

0-62 

64 

6 

6| 

0-75 

53 

7 

^1, 

0-87 

46 

8 

5" 

I 

40 

9 

4i] 

1-25 

32 

ID 

4 

1-50 

26J 

II 

3h 

1-75 

22i 

12 

3i 

2 

26 

13 

3 

2'25 

i7i 

14 

2f 

2.50 

16 

15 

2-§- 

2-75 

14 

16 

2J 

3 

13 

17 

2i 

3-50 

II 

18 

2i 

20 

2 

Testing  Lenses. — It  is  important  to  be  able  to  test  a 
lens  and  find  out  its  optical  value.  Instead  of  going 
through  the  process  of  finding  its  principal  focus,  and 
measuring  the  distance  of  this  from  the  lens  centre,  we 
place  in  front  of  it  lenses  of  the  opposite  value;  thus, 
if  we  wish  to  find  the  strength  of  a  convex  glass,  we 
neutralize  it  with  concave  glasses. 

A  finer  test  is  to  employ  the  parallactic  movement.  If 
we  look  at  a  distant  object  through  a  convex  glass  and 
move  the  glass,  the  object  appears  to  move  in  the  oppo- 
site direction;  if  we  use  a  concave  glass,  the  object 
appears  to  move  in  the  same  direction.  So  long  as 
there  is  any  movement  we  must  place  up  concave  or 
convex  glasses,  according  as  the  displacement  of  the 
object  is  "  against  "  or  "  with." 

In  testing  cylinders  we  have  to  ascertain  not  only  the 
value,  but  also  the  direction  of  the  axis. 


LENSES  15 

When  cylinders  are  moved  in  front  of  the  eye  in  the 
direction  of  the  axis,  objects  looked  at  through  them  are 
not  displaced;  but  the  smallest  rotation  of  the  cylinder 
causes  displacement,  which  reaches  its  maximum  when 
the  movement  of  the  cylinder  is  in  the  direction  at 
right  angles  to  its  axis.  In  this  position  neutralize  with 
cylindrical  lenses  of  the  opposite  value,  bearing  in  mind 
that  displacement  takes  place  "  against  "  the  movement 
so  long  as  a  convex  lens  predominates,  and  "  with  "  the 
movement  so  long  as  a  concave  one  predominates.  The 
axes  of  the  two  lenses  must  coincide. 

When  testing  a  sphero-cylindrical  glass,  the  spherical 
lens  should  be  first  neutralized. 

A  great  saving  of  time  is  effected  by  testing  glasses  with 
the  Geneva  lens  measure  and  the  Maddox  cylinder-axis 
finder. 

The  Combination  of  Lenses — Convex  Spherical  Lenses. 
— The  ordinary  way  for  such  lenses  to  be  ground  is  to 
work  half  the  power  needed  on  each  surface ;  thus,  when 
+  4*  is  required,  each  surface  of  the  lens  is  made  equal 
to  +2,  as  if  two  plano-convex  glasses  of  -1-2  had  their 
plane  surfaces  cemented  together. 

Another  method  of  working  a  convex  lens  is  to  grind 
the  surface  away  from  the  eye  as  a  convex  lens  of  higher 
power  than  is  required,  and  the  other  surface  concave  of 
such  a  strength  as  to  reduce  the  convex  surface  to  the 
desired  amount.  Thus,  when  -1-4  is  ordered,  one  surface 
can  be  made  +7  and  the  other  surface  -3,  or  one 
surface  can  be  -1-6  and  the  other  -2.  Such  lenses  are 
called  "  periscopic  *';  they  enlarge  the  field  of  distant 
vision,  as  the  eye  in  all  its  movements  is  at  the  same 
distance  from  the  surface  of  the  glass,  but  their  chief 
advantage  is  in  enabling  the  glasses  to  be  placed  nearer 
the  eye  without  being  touched  by  the  lashes. 

*  In  the  following  pages,  the  numeration  of  lenses  will  always 
be  in  dioptres,  and  "  d  "  after  the  numeral  will  be  generally 
omitted. 


l6  THE   REFRACTION   OF  THE  EYE 

Concave  Spherical  Lenses. — These  are  usually  made 
with  half  the  strength  required,  on  each  surface.  A 
certain  amount  of  periscopic  effect  may  be  obtained  by 
grinding  the  surface  nearer  the  eye  more  concave,  and 
reducing  the  other:  thus,  —8  may  be,  and  usually  is, 
made  —  4  on  each  surface,  or  the  surface  nearer  the  eye 
may  be  made  —6  and  the  other  —  2 ;  or  the  lens  may  be 
ground  as  a  diverging  meniscus  (Fig.  7,  f)  ;  thus,  if  -  3  is 
required,  the  surface  next  the  eye  is  ground  as  -5,  and 
the  other  surface  as  +2. 

Cylindrical  Lenses. — When  a  cylinder  only  is  pre- 
scribed, it  is  ground  on  one  surface,  the  other  surface 
remaining  plane.  When  combined  with  a  spherical  lens, 
it  is  usual  to  grind  the  sphere  on  one  surface,  and  the 
cylinder  on  the  other.  When  a  convex  sphere  and 
cylinder  are  required,  the  periscopic  effect  can  be  pro- 
duced by  grinding  a  concave  cylinder  on  the  surface 
nearer  the  eye,  and  increasing  the  spherical  strength  by 
the  amount  of  the  cylinder.  Thus,  supposing  +2  cyl. 
axis  vert,  c:;  +3  sph.  is  needed,  it  may  be  ordered  thus: 

—  2  cyl.  axis  horizontal  c^  +5  sph. 

In  ordering  glasses  for  mixed  astigmatism  (see  page 
137),  the  periscopic  effect  is  produced  by  combining  a 
convex  cyhnder  with  a  concave  sphere,  and  mounting  the 
latter  next  the  eye. 

As  it  is  very  important  to  divide  the  strength  of  the 
lens  between  the  two  surfaces  when  dealing  with  high 
powers,  a  cylinder,  if  also  required  in  such  a  case,  must 
be  worked  on  one  of  the  spherical  surfaces.  These 
lenses  are  called  Toric  lenses,  and  any  combination  of 
spherical  lens  (•25  to  18),  with  cyHndrical  lens  (from  •25 
to  3)  can  be  supplied.  Thus,  if  — 14  sph.  oj  -  2  cyl.  axis 
horizontal  is  required,  one  surface  is  made  -  7,  and  the 
other  -  7  sph.  c::>  -  2  cyl.  These  toric  lenses  are  very 
useful  in  high  myopia,  and  also  in  aphakia. 

Bi'Focal  Lenses  (see  Presbyopia,  page  151). — Where 


BI-FOCALS  17 

different  lenses  are  required  for  distance  and  reading, 
bi-focal  lenses  are  generally  prescribed. 

The  earliest  forms  of  these  glasses  were  straight  split 
bi-focals,  also  called  "  Franklin  "  glasses,  the  lenses  being 
two  separate  lenses  divided  horizontally  and  in  the 
middle.  Against  the  many  disadvantages  of  this  form 
of  bi-focal  was  the  one  advantage  that  the  lower  lens 
could  be  slanted  and  made  more  or  less  parallel  with  the 
book  or  paper  that  was  being  read. 

An  improvement  on  these  "  split  "  bi-focals  consisted 
having  the  reading  addition  cemented  on  by  Canada 
dlsam,  either  as  in  Fig.  15  or  as  a  small  round  wafer. 


Fig.  15. 

The  disadvantage  of  both  these  forms  was  chiefly 
manifested  by  the  dividing  line  between  the  lenses  being 
visible  and  constantly  causing  annoyance  to  the  wearer. 
Another  disadvantage  of  the  cemented  bi-focals  is  the 
tendency  of  the  balsam  to  dry  or  crystallize. 

The  next  improvement  did  away  with  the  visible  line, 
and  these  bi-focals  are  called  "  invisible  bi-focals."  They 
are  of  two  kinds,  one  the  so-called  "  Kryptok,"  where 
a  concavity  is  ground  in  the  lower  part  of  the  distance 
glass,  and  the  reading  glass,  of  a  higher  refractive  index, 
is  fused  into  it;  the  other,  the  final  perfected  form  of 
invisible  bi-focals  is  the  "  Luxe,"  which  consists  of 
one  glass  only.  The  glass  is  made  from  a  solid  piece 
of  crown  glass  with  the  two  lenses  ground  invisibly 
on  its  surface.    The  chief  advantage  of  these  "  Luxe  " 


l8  THE   REFRACTION   OF  THE   EYE 

bi-focals  is  that  the  centring  of  both  portions  is  more 
under  control,  and  there  is  no  chromatic  aberratioii, 
which  is  so  often  present  in  the  fused  form. 

Spherical  Aberration. — In  most  lenses  the  rays  passing 
through  the  peripheral  part  of  the  lens  do  not  focus  at 
the  same  spot  as  those  which  pass  through  the  central 
portion.  In  convex  lenses,  when  the  peripheral  rays 
focus  in  front  of  the  central  rays,  the  aberration  is  spoken 
of  as  positive ;  and  when  the  peripheral  rays  focus 
behind  the  central,  as  negative.  The  crystalline  lens 
suffers  from  spherical  aberration,  but  it  is  more  or 
less  hidden  by  the  contraction  of  the  pupil.  During 
mydriasis  this  aberration  may  interfere  considerably 
with  vision,  but  may  be  corrected  by  placing  in  the 
trial  frame  an  opaque  disc  with  a  central  circular  open- 
ing. If  the  refraction  is  being  estimated,  this  opening 
should  not  be  smaller  than  4  mm.  in  diameter. 


CHAPTER  II 

OPTICAL  PROPERTIES  OF  THE  NORMAL  EYE 

The  eye  is  constructed  in  the  form  of  a  photographic 
camera.  As  in  the  camera,  there  is  a  closed  darkened 
box  open  in  front,  where  there  is  an  arrangement  of 
lenses  to  focus  an  object  on  the  back,  at  which  spot 
there  is  the  apparatus  for  receiving  the  perfectly-formed 
image :  the  plate  in  the  camera,  the  retina  in  the  eye. 

As  in  the  camera,  there  are  two  conditions  which  must 
exist  in  the  eye :  firstly,  the  media  must  be  transparent ; 
and,  secondly,  the  focusing  must  be  so  arranged  that  a 
perfect  image  of  the  external  object  is  formed  on  the 
retina — i.e.,  the  principal  focus  of  the  eye  must  coincide 
with  the  retina. 

All  deviations  from  this  latter  condition  are  called 
errors  of  refraction  and  accommodation. 

The  Refraction  of  the  Normal  Eye  at  Rest— i.e.,  in 
the  Absence  of  any  Effort  of  the  Accommodation — 
Dioptric  Apparatus  of  the  Eye. — The  simplest  form  of  a 
dioptric  apparatus  is  when  two  media  of  different  refrac- 
tive power  are  separated  by  a  spherical  surface. 

Such  a  system  is  represented  by  Fig.  i6,  where 
X  y  z  is  di  spherical  surface  separating  a  less  refractive 
medium  on  the  left  from  a  more  refractive  medium  on 
the  right.  The  line  o  A  passing  perpendicularly  to  the 
surface  of  the  sphere  and  through  its  centre  at  N  is  called 
the  "  optic  axis." 

All  rays  passing  .^normally  to  the  surface,  such  as  R  N 
and  s  N,  like  the  optic  axis,  pass  through  n,  and  undergo 
19 


20  THE  REFRACTION   OF  THE   EYE 

no  refraction;  N,  the  centre  of  the  sphere,  is  called  the 
"  nodal  point." 

The  point  y  where  the  optic  axis  cuts  the  sphere  is 
called  the  "  principal  point." 

Rays  c,  d,  parallel  to  the  optic  axis  in  the  less  dense 
medium,  unite  somewhere  on  the  optic  axis  at  the  point 
F,  called  the  "  posterior  principal  focus." 

On  the  optic  axis,  in  the  less  dense  medium,  there  is 
another  point  (f'),  called  the  "  anterior  principal  focus," 
whence  divergent  rays  passing  into  the  denser  medium 
are  refracted,  and  become  parallel  to  the  optic  axis  as  at  zf. 


Fig.  i6. 

These  four  points — the  principal  point,  nodal  point, 
and  anterior  and  posterior  principal  focus — are  called 
"  cardinal  points  "  of  the  system. 

In  the  eye  the  system  is  much  more  complicated.  A 
ray  of  light  passing  into  the  eye  meets  the  following  sur- 
faces and  media  in  the  order  named:  Anterior  surface 
of  the  cornea,  substance  of  the  cornea,  posterior  surface 
of  the  cornea,  aqueous,  anterior  surface  of  the  lens,  sub- 
stance of  the  lens,  posterior  surface  of  the  lens,  and 
vitreous.  Thus  there  are  four  surfaces  and,  if  we  include 
the  air,  four  media.  As  the  anterior  and  posterior  sur- 
faces of  the  cornea  are  parallel,  we  may  neglect  the  sub- 
stance of  the  cornea,  and  consider  the  two  surfaces  as 
one.     Again,  as  the  indices  of  refraction  of  the  aqueous 


THE  CARDINAL   POINTS  21 

and  vitreous  are  identical  (see  page  4),  we  may  assume 
them  to  be  one  medium.  In  this  manner  the  eye  is 
reduced  to  three  surfaces  and  three  media. 

These  three  surfaces — the  cornea,  and  the  anterior  and 
posterior  surfaces  of  the  lens — are  symmetrically  centred 
round  the  optic  axis  of  the  whole  system,  which  may 
now  be  reduced  to  a  compound  system,  consisting  of  the 
cornea  and  a  bi-convex  lens ;  we  find  the  principal  points 
p'  p"  (Fig.  17)  and  the  nodal  points  n'  n"  of  the  cornea 
and  lens ;  and,  finally,  take  the  mean  of  these  two  points, 


Fig.  17. 

and  get  p  the  principal  focus  and  N  the  nodal  point. 
Such  an  eye  is  known  as  the  "  reduced  eye,"  and  was 
suggested  by  Listing. 

The  positions  of  the  cardinal  points  of  the  reduced  eye 
are — 

Principal  point,  in  the  aqueous,  2 '3448  mm.  behind  the 
anterior  surface  of  the  cornea. 

Nodal  point,  in  the  lens,  '4764  mm.  from  its  posterior 
surface,  and  about  15  mm.  from  the  retina. 

Posterior  principal  focus,  22 •81 9  mm.  behind  the  an- 
terior surface  of  the  cornea — i.e.,  on  the  retina  of  the 
normal  eye.     (This  is  the  length  of  the  standard  eye.) 

Anterior  principal  focus,  12 '8  mm.  in  front  of  the 
anterior  surface*  of  the  cornea. 


22  THE  REFRACTION  OF  THE  EYE 

The  principal  plane  r  p  s  (Fig.  i8)  is  where  the  one 
surface  of  this  reduced  system  passes  through  the  prin- 
cipal point  p  which  is  considered  the  centre  of  refraction 
of  the  eye. 

The  optic  axis  (o  a)  is  an  imaginary  line  passing 
through  the  centre  of  the  cornea  and  the  nodal  point, 
and  meeting  the  retina  a  little  above  and  to  the  nasal 
side  of  the  fovea. 

The  nodal  point  corresponds  to  the  optical  centre, 
and,  as  we  have  already  seen,  all  rays  passing  through 
it  are  unrefracted. 

We  can  now  ascertain  how  an  image  is  formed  on  the 
retina. 

Let  X  Y  (Fig.  i8)  be  an  object  in  front  of  the  eye;  each 


Fig.  I 8. 


point  of  this  sends  out  a  pencil  of  divergent  rays,  and  all 
those  which  pass  into  the  eye,  by  the  dioptric  system, 
are  made  to  converge  into  a  point  on  the  retina. 

Each  pencil  of  rays  from  the  point  x  has  a  principal 
ray  x  a,  which  is  normal  to  the  surface,  and,  passing 
straight  through  the  nodal  point  n  without  refraction, 
impinges  on  the  retina  at  x'.  The  other  rays  from  x  are 
increasingly  divergent,  and  are  represented  by  x  6,  x  c; 
they  undergo  refraction,  and  converge  together  at  some 
point  on  the  principal  ray,  which  in  the  normal  eye  will 
be  at  x'.  In  like  manner  we  can  trace  the  rays  from  the 
other  extreme  point  Y,  which  forms  an  image  at  y',  and 
so  for  all  the  other  points. 

In  tracing  the  formation  of  an  image  on  the  retina. 


SIZE   OF  RETINAL   IMAGE  23 

we  can  ignore  all  the  rays  from  a  point  of  the  object, 
except  the  principal  ray,  which  we  trace  through  the 
nodal  point;  and  by  tracing  all  the  luminous  points 
from  an  object  through  the  nodal  point,  we  obtain  in 
the  normal  eye  an  inverted  image  of  the  object  on  the 
retina. 

The  nearer  the  object  is  to  the  eye,  the  larger  will  be 
its  image,  and  vice  versa.  The  size  of  the  retinal  image 
is  therefore  directly  proportional  to  the  distance  of  the 
object  from  the  eye. 

It  is  sometimes  important  for  the  oculist  to  determine 
the  size  of  the  retinal  image  of  an  object  in  order  to  dis- 


cover the  size  of  a  diseased  area;  this  can  be  estimated 
if  the  size  of  the  object  and  its  distance  from  the  eye  be 
known. 

The  triangles  A  N  B  and  a  ^  h  (Fig.  19)  are  similar, 
hence  ah  :  hB  :  :  «  N  :  A  n — that  is,  the  size  of  the  area 
on  the  retina  is  to  the  size  of  the  object  as  the  distance 
from  the  nodal  point  to  the  retina  is  to  the  distance 
of  the  nodal  point  from  the  object.  Let  the  latter  be 
10  metres  and  the  size  of  the  object  i  metre;  we  know 
that  the  distance  a;  N  is  15  mm. — consequently : 

ah  :  1,000  :   :  15  :  10,000; 

,     15,000 

.-.  ah  =  ^^ =1*5  mm. 

10,000 


24  THE  REFRACTIOnIoF  THE  EYE 

The  perfect  type  of  eye  is  that  in  which  the  retina 
coincides  with  the  posterior  principal  focus,  and  is  called 


Fig.  20. 

Showing  parallel  rays  focused  on  the  retina  in  emmetropia  (e), 
behind  the  retina  in  hyperopia  (h),  and  in  front  of  the  retina 
in  myopia  (m)  . 

the  "  emmetropic  "  eye  (e,  Fig.  20),  and  any  deviation 
from  this  is  called  ametropia. 


VISUAL  ANGLE 


^5 


The   following   table  gives   an   idea  of   the  relative 

frequency  of  the  different  forms  of  ametropia : 

(a)  Emmetropia  (see 
Presbyopia   be- 


2500  individuals 
whose  sight 
after  correc- 
tion was  nor- 
mal and  who 
had  no  disease 
of  the  eyes. 


:)  Same  refrac- 
tion in  both 
eyes  (O57) 


low) 

9 

(b)    Hypermetropia  . 

^3 

(c)    Myopia 

22 

{d)  Astigmatism — 

Hypermetropic 

43« 

Myopic     . 

113 

Mixed       . 

12 

U^) 


Refraction  different  in  the  two  eyes 
(Anisometropia)     .         .         .         . 


^843 


2500 


5000  eyes  (as  above) — 

Emmetropia  ,  .......         56 

Hypermetropia       .......       425 

Myopia  ........       216 

Astigmatism  .         .         .         .         .         .         .         .4303 

5000 

Of  the  2500  individuals,  961  were  presbyopic,  and  only  9  of 
these  were  emmetropic. 

If  the  posterior  principal  focus  is  beyond  the  retina, 
the  eye  is  too  short,  and  parallel  rays,  when  they  meet 
the  retina,  have  not  yet  come  to  a  focus,  and  only  con- 
vergent rays  come  to  a  focus.  This  is  called  "  hyper- 
opia "  (h,  Fig.  20). 

If  the  principal  focus  is  in  front  of  the  retina,  the  eye 
is  too  long ;  parallel  rays  focus  in  front  of  the  retina,  and 
only  divergent  rays  focus  on  the  retina.  This  condition 
is  called  "  myopia  "  (m,  Fig.  20). 

The  Visual  Angle  and  Visual  Acuity. — Rays  of  hght, 
proceeding  from  the  two  extremes  of  an  object  into  the 
eye,  meet  at  the  nodal  point  n  (Fig.  21)  before  crossing 
and  forming  the  inverted  image  on  the  retina,  and  the 
angle  included  at  n  is  called  the  "  visual  angle."  A  n  b 
is  the  visual  angle  of  the  object  a  b  (Fig.  21). 

The  size  of  the  visual  angle  depends  on  the  size  of  the 
object  and  its  distance  from  the  eye;  thus,  a'  b',  which 


26 


THE   REFRACTION   OF   THE   EYE 


is  the  same  size  as  A  b,  subtends  a  larger  angle,  and  the 
image  is  larger;  and,  again,  a"  b"  subtending  the  same 
visual  angle  as  A  b  would  appear  to  be  the  same  size, 
whereas  it  is  much  smaller.  Fortunately,  we  do  not 
gain  our  estimation  of  the  size  of  objects  by  the  visual 
angle  alone;  experience  and  comparisons  with  other 
objects  of  known  size  are  brought  into  play,  and  enable 
us  to  correct  any  erroneous  judgment. 

The  smallest  visual  angle  in  which  the  standard  eye 
can  recognize  an  object  is  an  angle  of  one  minute,  so 
that  two  points  of  light,  such  as  two  stars,  separated  by 
an  angular  interval  of  less  than  one  minute  would 
appear  on  the  retina  as  only  one  point. 


Fig.  21. 


Test  Types. — It  is  most  important  to  have  a  standard 
measure  for  acuteness  of  vision,  and  Snellen  has  arranged 
test  types  on  such  a  plan  that  each  letter  is  made  up  of 
several  parts,  each  of  such  a  size  that  it  subtends  an 
angle  of  one  minute  vertically  and  horizontally,  the 
whole  letter  subtending  an  angle  of  five  minutes  verticallj* 
and  horizontally  when  read  at  the  standard  distance. 

Thus,  in  Fig.  22,  the  F  is  made  out  of  twenty-five 
squares,  each  subtending  an  angle  of  one  minute  (the 
whole  letter  subtending  an  angle  of  five  minutes)  when 
read  by  the  normal  eye  at  12  metres;  and  the  l,  which 
is  constructed  on  the  same  plan,  subtends  the  same 
angle  when  read  by  the  normal  eye  at  6  metres. 

The  numbers  of  the  different-sized  letters  in  Snellen's 
types  represent  the  distance  in  metres  at  which  the 


VISUAL   ACUITY  27 

standard  eye  can  read  them;  in  other  words,  at  that 
distance  they  subtend  an  angle  of  five  minutes.  For 
instance,  the  largest  type,  d  =  60  (see  type  at  end  of 
book),  can  be  read  by  the  normal  eye  at  60  metres,  and 
it  subtends  the  same  angle  as  the  type  d  =  24  read  at 
24  metres,  and  d  =^  6  read  at  6  metres.  The  acuteness 
of  vision  is  represented  by  a  fraction  which  has  for  its 
numerator  the  distance  in  metres  at  which  the  type  is 
read,  and  for  its  denominator  the  distance  at  which  it 
ought  to  be  read.  The  line  d  =  6  means  that  this  type 
can  be  read  by  the  normal  eye  at  6  metres,  and  if  the 
patient  under  examination  can  read  it  at  6  metres,  the 
fraction  is  | — that  is,  normal  vision.  If  the  patient 
cannot  see  a  smaller  type  than  d  =  12  at  6  metres,  his 


Fig,  22. 


vision  ==  tV  ;  if  D  =  60  is  the  only  letter  that  can  be  read 
at  6  metres,  his  vision  =  A — i-c-,  one- tenth  of  the 
normal.  If  d  =  60  cannot  be  read  at  6  metres,  the 
patient  must  be  made  to  approach  the  type;  if  he  can 
just  read  this  letter  at  2  metres,  his  vision  is  i~^',  he  has 
only  one-thirtieth  of  normal  vision.  Although  J  is  the 
standard  of  normal  acuteness  of  vision,  many  eyes  can 
see  better — viz.,  |,  or  even  J;  i.e.,  such  eyes  can  read  at 
6  metres  type  that  the  standard  eye  cannot  read  at  a 
greater  distance  than  5  and  4  metres  respectively. 

If  the  visual  acuity  is  so  lowered  that  the  patient 
cannot  see  any  letter  at  any  distance,  it  can  be  measured 
by  finding  whether  he  can  count  fingers,  and  if  so,  at 
what  distance,  and  failing  this,  by  finding  whether  he 


2S  THE    REFRACTION   OP  THE  EYE 

can  distinguish  between  black  and  white.  If  vision  is 
even  worse  than  this,  we  take  him  to  the  Hght  and  pass 
the  hand  in  front  of  the  eye — i.e.,  between  the  eye  and 
the  Hght ;  if  movement  is  recognized,  we  find  out  whether 
he  can  distinguish  the  direction  of  the  movement. 

Finally,  if  he  fails  at  all  these  tests,  he  should  be 
taken  into  the  dark  room,  and  a  strong  beam  of  light 
should  be  directed  on  to  the  eye;  if  this  is  not  perceived, 
vision  =  o;  if  it  is  perceived,  we  ascertain  whether  he 
has  good  projection,  by  reflecting  the  light  on  to  the  eye 
from  different  positions,  and  ascertaining  whether  he 
can  tell  whence  the  light  is  coming. 

Type  for  Near  Vision. — As  the  "  Schrift-scalen  "  of 
Professor  Jaeger  represent  no  standard,  this  type  is  being 
superseded  by  Snellen's,  which  is  on  the  same  principle 
as  his  distant  type,  the  figure  over  the  type  signifying  the 
greatest  distance  at  which  the  normal  eye  can  read  it, 
and,  of  course,  subtending  an  angle  of  five  minutes  at 
that  distance.  The  sizes  range  from  d  =  '5  to  d  =  4 
(see  type  at  end  of  book),  j  '2  (Jaeger)  is  the  equivalent' 
of  D  'S  (Snellen). 


D  =  0,5. 

JU  to  my  boat,  it  was  a  very  good  one,  and  that  he  saw,  and  told  me  he  would  buy  it  of 
me  for  the  ship's  om  and  asked  me  what  I  would  hare  for  it. 

D  =  0,6. 

In  this  distress  the  mate  of  our  vessel  la3rs  hold  of  the  boat,  and  with  the  help 
of  the  rest  of  the  men,  they  got  her  slung  over  the  ship's  side. 

D  =  0,8. 

A  little  after  noon  I  found  the  sea  very  calm,  and  the  tide  ebbed  so 
far  out,  that  I  could  come  within  a  quarter  of  a  mile  of  the  ship  ; 

D  =  l. 

In  search  of  a  place  proper  for  this,  I  found  a  little  plain 
on  the  side  of  a  rising  hill,  whose  front  towards  this  little 
plain  was  steep  as  a  house  side. 

D  =  l,25. 

Then  I  took  the  pieces  of  cable  which  I  had  cut 
in  the  ship,  and  laid  them  in  rows  one  upon 
another,  within  the  circle  between  these  two  rows 
of  stakes. 

D  =  l,5. 

When  I  had  done  this,  I  began  to  work 
my  way  into  the  rock,  and  bringing  all 
the  earth  and  stones,  that  I  dug  down, 
out  through  my  tent. 

D=2,25. 

For  in  this  way  you  may 
always  damp  our  ardour. 

D  =  3. 

I  sai^T  no  one  there. 

For  the  ensuiner 


D=60f200) 


D-36  (120) 


D=24  (80) 


D=I8(60) 


D-12  (40) 


DF  O  E 


D=9 (30) 


G  L  Z  T  O 

D«6  (20) 

L  T   R  F  P 

D-5  (16) 

A  P  O   R  F    D 


CHAPTER  III 

ACCOMMODATION 

When,  with  one  eye  closed,  the  other  eye  focuses  a 
needle  a  metre  from  the  eye,  another  needle  placed  half 
a  metre  from  the  eye  will  appear  blurred. 

If  A  (Fig.  23)  be  the  first  needle,  a  clear  image  is 
formed  by  the  exact  focusing  of  it  on  the  retina  at  a', 


Fig.  23. 


Fig.   24. 

while  the  image  of  B  will  be  focused  beyond  the  retina 
at  b',  the  rays  from  B  impinging  on  the  retina  in  the 
form  of  a  collection  of  diffusion  circles. 

On  the  other  hand,  if  the  needle  b  be  focused  on  the 
retina — i.e.,  if  its  image  be  clearly  seen — the  needle  A 


30  THE   REFRACTION   OF  THE   EYE 

will  appear  hazy  or  out  of  focus,  because  its  image  is 
focused  in  front  of  the  retina,  and,  after  crossing,  the 
rays  impinge  on  the  retina  as  diffusion  circles. 

Diffusion  Circles. — Two  points  of  light,  if  near  one 
another  and  out  of  focus,  appear  as  two  diffusion  circles 
overlapping  each  other  if  near  enough  (Fig.  24,  a). 

As  a  line  in  focus  may  be  considered  to  be  an  infinite 
number  of  points  of  light  in  focus,  so  a  line  out  of  focus 
consists  of  a  series  of  overlapping  diffusion  circles 
(Fig.  24,  b)  which  makes  the  line  appear  as  a  broad 
band,  as  Fig.  24,  c.  The  further  the  rays  focus  from  the 
retina,  the  larger  will  be  the  diffusion  circles.  In  Fig.  25 
both  A  and  b  are  "  out  of  focus,"  but  b  is  more  so  than 
A,  and  consequently  the  diffusion  circles  formed  by  b 


Fig.  25. 

occup3  a  larrer  area;  and,  again,  the  larger  the  pupil  the 
larger  the  area  of  diffusion  circles,  because  as  the  pupil 
contracts  it  cuts  off  the  outside  rays. 

The  alteration  of  the  eye  by  its  focusing  mechanism 
is  called  accommodation.  The  photographer  focuses 
by  lengthening  or  shortening  the  distance  between  the 
back  of  the  camera  and  the  lens,  but  he  could  also  focus 
by  adding  a  convex  or  concave  lens  to  that  he  is  already 
using. 

It  is  in  this  latter  way  that  the  eye  focuses;  the  eye 
cannot  lengthen,  but  the  lens  can  become  more  convex, 
which  has  the  same  result  as  adding  a  convex  lens. 

In  the  normal  standard  eye,  parallel  rays,  coming  from 
a  distance  beyond  6  metres,  are  focused  on  the  retina 
when  the  eye  is  at  rest — i.e.,  when  the  apparatus  of 


ACCOMMODATION  31 

accommodation  is  not  being  used;  but  when  the  eye 
wishes  to  see  clearly  any  object  nearer  than  6  metres, 
the  lens  must  become  more  convex. 

After  looking  at  the  needle  A,  when  we  look  at  needle 
B  and  obtain  a  clear  image,  we  are  distinctly  conscious 
of  an  effort,  and  the  nearer  we  approach  b  to  the  eye 
the  greater  is  the  effort,  till  we  reach  a  spot  near  the  eye 
when  no  effort  will  produce  a  clear  image,  because  the 
rays  from  the  needle  are  too  divergent  to  be  focused  on 
the  retina.  The  nearest  point  to  the  eye  at  which  the 
object  is  recognized  as  a  perfectly  clear  image  is  called 
the  near  point  "P."  After  looking  at  the  needle  close 
to  the  eye,  and  again  looking  at  the  distant  needle,  we 
are  conscious  of  a  relaxation  of  our  efforts. 

How  do  we  know  that  this  focusing  or  accommodation 
is  caused  by  an  increased  convexity  of  the  lens  ? 

The  Mechanism  of  Accommodation. — If  we  take  a 
patient  into  the  dark  room  and  hold  a  candle  in  front  o 
the  eye  a  little  to  one  side,  we  shall  see  three  images  oi 
this  candle  in  the  eye.  One,  the  brightest,  is  upright, 
the  reflection  coming  from  the  anterior  surface  of  the 
cornea;  the  second,  duller,  is  also  upright,  and  is  the 
reflection  from  the  anterior  surface  of  the  lens;  and  the 
third  is  inverted,  duller,  and  smaller,  and  is  from  the 
posterior  surface  of  the  lens.  The  patient  is  told  to  look 
into  distance,  and  the  size  and  position  of  these  images 
is  noted,  and  then,  carefully  watching  them,  he  is  told  to 
gaze  at  a  near  point.  No  change  will  be  seen  in  the  first 
image  (proving  the  fallacy  of  the  old  theory  that  the 
cornea  becomes  more  convex  during  accommodation), 
and  little  change  in  the  third;  but  the  middle  image — 
viz.,  that  from  the  anterior  surface  of  the  lens — becomes 
distinctly  smaller  and  moves  forward,  showing  that  this 
surface  has  become  more  convex. 

In  accommodation,  then,  the  lens  becomes  larger  in 
its  antero-posterior  diameter,  and  as  it  does  not  alter  in 
volume,  it  becomes  narrower  in  its  equatorial  dimensions. 


32 


THE   REFRACTION   OF  THE   EYE 


Fig.  26. — Diagrammatic    Section    of   the    Ciliary    Region 
OF  the  Eye. 

C,  Cornea;  c  S,  Schlemm's canal;  O  s,  era  serrata;  / p,  pectinated 
ligament ;  e  F,  Fontana's  space ;  T,  tendinous  ring ;  m,  merid- 
ional fibres;  r,  radiating  fibres;  c,  circular  fibres  of  the 
ciliary  muscle;  Z,  zone  of  Zinn. 

The  full  lines  indicate  the  lens,  iris,  and  ciliary  body  at  rest,  and 
the  dotted  lines  the  same  in  a  state  of  accommodation. 
(Reduced  from  Landolt.) 


ACCOMMODATIOK  33 

We  will  now  inquire  how  this  change  is  brought  about. 

According  to  Iwanoff,  the  ciliary  muscle  arises  from  a 
tendinous  ring  (Fig.  26,  t)  close  to  the  insertion  of  the 
iris  and  Schlemm's  canal  {c  S),  at  the  posterior  surface 
of  the  sclerotic,  close  to  its  junction  with  the  cornea. 
The  muscle  then  passes  backwards,  and  may  be  divided 
into  three  parts:  (i)  The  outermost  part  or  meridional 
portion,  passing  into  the  posterior  tendon  (m),  to  be 
inserted  into  the  choroid;  (2)  the  radiating  portion  (r) ; 
and  (3)  the  annular  portion  or  circular  muscle  of  Miiller 
(c),  passing  directly  backwards  and  inwards  respectively, 
to  be  inserted  into  an  agglomeration  of  fibres  called  the 
"  zone  of  Zinn  "  (Z).  These  fibres  arise  partly  from  the 
ciliary  portion  of  the  retina  at  the  ora  serrata  (0  s) ,  and 
partly  from  the  ciliary  processes  and  the  intervals 
between  them,  and  they  pass  forwards  and  backwards, 
to  be  inserted  into  the  anterior  and  posterior  capsule  of 
the  lens. 

The  annular  muscle  of  Miiller  is  a  sphincter,  and  does 
the  principal  work;  hence  it  is  always  larger  in  hyperopia, 
because  of  the  extra  accommodation  work  necessary,  and 
is  badly  developed  in  myopia. 

There  are  two  theories  as  to  the  modus  operandi  of  the 
ciliary  muscle  when  accommodating.  The  old  theory 
started  by  Helmholtz,  and  supported  by  Hess,  is  as 
follows : 

When  the  ciliary  muscle  contracts,  it  pulls  forwards 
and  inwards  the  capsule  of  the  lens,  the  inward  pull 
being  specially  brought  about  by  the  contraction  of  the 
circular  muscle  of  Miiller.  The  contraction  of  the  longi- 
tudinal fibres  pulls  forward  the  choroid  and  the  portion 
of  the  ciliary  body  near  it. 

By  this  process,  they  contend,  the  tension  on  the  lens 
capsule  is  relaxed,  and  the  lens,  which  has  been  in  a 
state  of  compression,  is  allowed  to  assume  a  more  convex 
form. 

The  new  theory  advanced  by  Tscherning  maintains 

3 


34  THE   REFRACTION   OF  THE   EYE 

that  the  action  of  the  ciliary  muscle  is  to  increase  the 
tension  on  the  fibres  of  the  suspensory  ligament,  and  to 
alter  the  lens  from  a  spherical  to  a  hyperboloid  form, 
and  this  theory  is  founded  on  the  work  of  Thomas 
Young.  According  to  this  theory,  the  lens  becomes 
more  conical  under  accommodation,  and  the  contraction 
of  the  pupil,  that  occurs  at  the  same  time,  masks  the 
increased  aberration  which  results  from  the  flattening 
of  its  periphery. 

The  posterior  surface  of  the  lens  does  become  slightly 
mere  convex  during  accommodation,  but  it  does  not 
change  its  position,  the  increase  of  thickness  of  the  lens 
being  effected  by  the  advance  of  the  anterior  surface. 

Tscherning's  theory  of  accommodation  is  entirely  supported 
clinically.  Under  the  Helmholtz  theory  it  is  difficult  to  under- 
stand the  possibility  of  meridional  asymmetrical  accommodation, 
and  as  difficult  to  believe  in  the  possibility  of  obtaining  20  d  of 
accommodative  power  which  is  frequently  seen  in  young  subjects. 
Lastly,  the  Helmholtz  theory  is  totally  against  the  idea  of  rest. 

Amplitude  of  Accommodation. — At  rest,  the  eye  is 
adapted  for  the  most  distant  point  it  can  see  distinctly — ■ 
viz.,  its  punctum  remotum  (R) ;  while  the  greatest  possible 
contraction  of  the  ciliary  muscle  adapts  the  eye  to  the 
nearest  point  it  can  see  distinctly — viz.,  its  punctum 
proximum  (P),  which  represents  the  greatest  possible 
contraction.  The  force  required  to  change  the  eye  from 
R  to  P  is  called  the  "  amplitude  of  accommodation,"  and 
is  represented  by  the  difference  between  the  refraction  of 
the  eye  at  rest  and  the  refraction  when  doing  its  utmost 
work. 

Donders  represented  the  equation  thus: 

III 
A~P~R 

or  '  a  =  p-r. 

Where  "  a  "  equals  the  numbers  of  dioptres  represented 
by  the  accommodation,  "  p  "  equals  the  numt^er  of  diop- 


ACCOMMODATION  35 

tres  represented  by  the  eye  when  in  a  state  of  maximum 
refraction—/.^.,  when  adapted  for  its  nearest  distinct 
point — and  "  r  "  equals  the  number  of  dioptres  repre- 
sented by  the  eye  at  rest — i.e.,  when  adopted  for  its 
furthest  distinct  point.  In  other  words,  "  r  "  represents 
the  static  refraction  of  the  eye. 

In  emmetropia,  as  R  is  at  "  infinity,  "  r  "  can  be 
ignored, 

.-.a  =  p. 

Therefore  the  ampHtude  of  accommodation  is  represented 
by  the  nearest  distinct  point ;  if  this  is  9  cms.  off,  "  a  "  = 
-f-  =  II — that  is,  the  power  of  accommodation  is  equal 
to  a  lens  of  eleven  dioptres. 

In  myopia  •'  r  "  has  a  positive  value.  Take,  for  ex- 
ample, a  person  whose  furthest  distinct  point  with  the 
eye  at  rest  is  33  cms.  (that  is,  a  myope  of  3),  and  sup- 
pose that  his  nearest  distinct  point  is  7  cms. ;  then 

a  =  p  -  r 

100       100 
=  -7 5^ 

=  14-3 
=  II. 

In  other  words,  14  would  represent  his  amplitude  of 
accommodation  if  he  were  emmetropic;  but  being 
myopic  to  the  extent  of  3,  we  must  subtract  that,  which 
leaves  us  11  to  represent  his  amplitude. 

In  hyperopia,  as  we  shall  see  later,  "  r  "  is  negative; 
therefore  the  equation  is 


a  =  p  -  (-^r) 
=  p  +r. 

Thus,  an  eye  hyperopic  to  the  extent  of  5,  having  its 
near  point  at  25  cms.  from  the  eye,  has  an  amphtude  of 
accommodation  equal  to  a  lens  of  9.    To  see  25  cms. 


36  THE  REFRACTION  OF  THE  EYE 

off,  the  eye  requires  an  accommodation  of  4  (-V/),  but 
it  has  already  expended  5  for  distance,  so  that 

a  =  p  -  {-  r) 
=  4  -  (-  5) 
=  4+5  =  9- 

We  thus  see  that  to  determine  the  ampHtude  or  range 
of  accommodation  we  must  find  R  and  P. 

R  is  represented  by  the  refraction  of  the  eye  at  rest. 

P  we  find  as  follows : 

Take  a  tape  graduated  on  one  side  in  centimetres,  and 
on  the  other  in  corresponding  dioptres ;  the  zero-end  of 
the  tape  is  attached  to  the  handle  of  a  frame,  into  which 
may  be  introduced  either  a  perforated  diaphragm  or  a 
paper  with  fine  print  upon  it,  or  threads  or  hairs ;  or  the 
ordinary  near  vision  test  card  and  separate  measure  may 
be  used.  The  test  object  is  brought  towards  the  eye 
under  examination  (the  other  one  being  covered)  until  it 
begins  to  appear  indistinct ;  we  then  read  off  on  the  tape 
the  distance  of  P  from  the  eye,  and  the  corresponding 
dioptres  (p)  representing  the  maximum  refractive  power 
of  the  eye. 

If  from  any  cause,  such  as  presbyopia  or  high 
hyperopia,  the  patient's  near  point  is  so  far  that  the 
above  tests  cannot  be  employed,  we  place  in  front  of 
the  eye  such  a  convex  glass  as  will  bring  the  punctum 
proximum  (P)  closer,  and  enable  him  to  read  d  =  .5  or 
see  the  words  in  the  frame,  such  glass  to  be,  of  course, 
deducted  afterwards.  Thus,  supposing  a  person  with 
+  2  can  bring  the  test  object  up  to  25  cms.  and  no  nearer, 
we  read  off  on  the  other  side  of  the  tape  4,  and  we 
subtract  the  +  2  from  this,  which  gives  us  p  =  2 — that 
is,  P  is  50  cms.  off.  If  he  is  an  emmetropic  presbyope, 
this  represents  his  amplitude  of  accommodation.  If  he 
is  hyperopic  to  the  extent  of  6,  then 

a  =  2  +  6 
=  8. 


ACCOMMODATION  37 

Or  suppose  the  patient,  being  hyperopic  and  presbyopic, 
requires  +5  to  read  at  33  cms.,  if  his  hyperopia  =  6,  then 

a  =  p  +  r 

=  (3  -  5)  +  6 
=  4- 

We  can  also  find  the  ampHtude  of  accommodaf ion  by 
ascertaining  the  strongest  concave  glass  the  patient  can 
"  overcome."  In  emmet ropia  such  glass  represents  the 
amplitude  of  accommodation.  In  hyperopia  the  amount 
of  hyperopia  must  be  added,  and  in  myopia  the  amount 
of  myopia  must  be  deducted. 

As  an  example,  we  find  a  patient  who  is  hyperopic  to 
the  extent  of  2  can  still  read  J  with  -  4,  but  he  cannot 
do  so  with  -  5;  thus  his  amplitude  of  accommodation 
is  4  +  2  =  6. 

It  necessarily  follows  that  to  determine  the  amplitude  of 
accommodation  of  an  eye,  its  refraction  must  be  accurately 
ascertained  and  the  patient  must  wear  the  full  correction  of 
the  error  when  the  examination  is  made. 

The  Region  of  Accommodation  is  quite  different  from 
the  range,  and  gives  very  little  idea  of  the  work  done. 

1      —————— -^ 

WD 
a^  WD 


00                                                                          R'lO'^'"     P'-S'" 
i      _ . 

lOD  2.0D 

a  =   too 
Fig.   27. 

Thus,  the  region  of  accommodation  in  an  emmetropic 
eye,  as  Fig.  27  (i),  is  from  infinity  (R)  to  10  cms.  (P)  in 


38  THE   REFRACTION   OF   THE   EYE 

front  of  the  eye,  while  in  Fig.  27  (2),  a  myopic  eye,  it  is 
only  from  10  cms.  (R)  to  5  cms.  (P)  in  front  of  the  eye, 
and  yet  in  each  case  the  same  amount  of  accommodation 
work  is  done,  which  is  equal  to  a  lens  of  10. 

Accommodation  is  spoken  of  as  absolute,  binocular, 
and  relative. 

Absolute  accommodation  is  the  full  amount  of  accom- 
modation of  one  eye,  the  other  being  excluded. 

Binocular  accommodation  is  the  full  amount  of  accom- 
modation which  both  eyes,  converging,  can  exert 
together. 

Relative  accommodation  is  the  limit  within  which 
accommodation  may  be  increased  or  decreased,  the  con- 
vergence remaining  the  same  (see  Convergence,  page  54). 


CHAPTER  IV 
CONVERGENCE 

Anatomical  and  Physiological  Considerations. — The  orbit  con- 
tains the  eyeball,  the  optic  nerve,  muscles,  lachrymal  gland 
vessels  and  nerves,  and  a  quantity  of  fat.  These  structures  are 
all  firmly  connected  by  a  system  of  fasciae.  Surrounding  the 
eyeball,  these  fasciae  are  condensed  in  a  fibrous  capsule — the 
fascia  bulbi  or  Tenon's  capsule.  This  capsule  consists  of  an 
external  capsule  and  an  internal  capsule.  It  is  perforated  by  the 
muscles  just  before  their  insertion  into  the  globe,  and  its  reflection 
unites  with  the  cone  of  fascia  surrounding  the  muscle ;  prolonga- 
tions and  thickenings  of  the  orbital  fasciae  of  these  muscles  are 
inserted  into  the  margins  of  the  orbit,  and  constitute  the  check 
ligaments.* 

The  muscles  which  move  the  eye  are  six  in  number,  and, 
with  the  exception  of  the  inferior  oblique,  which  arises  from 
the  anterior  and  inner  part  of  the  floor  of  the  orbit,  they  all 
arise  from  the  apex  of  the  orbit.  These  muscles  may  be  con- 
sidered as  three  pairs,  each  pair  rotating  the  eye  round  a  par- 
ticular axis.  The  four  recti — viz.,  superior,  inferior,  internal, 
and  external — -pass  forwards,  pierce  Tenon's  capsule,  from  which 
they  receive  a  sheath,  become  tendinous,  and  are  inserted  into 
the  sclerotic  not  far  from  the  margin  of  the  cornea,  the  most 
anterior  insertion  being  that  of  the  internal  rectus,  which  is  about 
6  mm.  from  the  margin  of  the  cornea.  The  superior  oblique 
passes  forwards  to  the  upper  and  inner  angle  of  the  orbit,  where 
it  becomes  temporarily  tendinous,  and  passes  through  a  pulley, 
after  which  it  becomes  muscular  again,  and  changes  its  direction, 
passing  backwards  and  outwards  through  Tenon's  capsule  to  be 
inserted  (tendinously)  into  the  sclerotic,  at  the  back  and  upper 
part  of  the  eye.  The  inferior  oblique  passes  outwards  and  back- 
wards, underneath  the  inferior  rectus,  and  then  between  the 
external  rectus  and  the  eye,  to  be  inserted  into  the  outer,  pos- 
terior, and  lower  part  of  the  eyeball,  not  very  far  from  the 
entrance  of  the  optic  nerve. 

The  axis  of  rotation  of  the  internal  and  external  recti  is  ver- 
tical, and  that  of  the  superior  and  inferior  recti  horizontal,  with 
the  inner  extremity  more  forward  than  the  outer  (Fig.  28) .     That 

*  See  Maddox,  "  Ocular  Muscles,"  1907,  page  26, 
39 


40 


THE  REFRACTION   OF  THE   EYE 


of  the  oblique  muscles  lies  also  in  the  horizontal  plane,  with  its 
anterior  extremity  tilted  outwards. 

The  movements  of  the  eyeball  are  produced  by  the  association 
of  various  muscles,  as  shown  below: 

I.  Elevation. — The  movement  of  the  eye  straight  up  is  pro- 
duced by  the  superior  rectus  and  inferior  oblique,   probably 


obi  6UP, 


Fig.  28. — Diagram  of  the  Attachments  of  the  Muscles 
OF  the  Left  Eye  and  of  their  Axes  of  Rotation  as 
seen  from  Above.     (Michael  Foster.) 

The  attachments  of  the  muscles  are  shown  by  the  beginning  of 
the  thick  lines,  and  the  direction  of  the  pull  is  shown  by  the 
arrows,  v  x  represents  the  visual  axis,  and  h  h  a  line  at 
right  angles  to  it. 

The  axis  of  rotation  of  the  internal  and  external  recti,  being 
perpendicular  to  the  plane  of  the  paper,  is  not  represented ; 
that  of  the  other  muscles  is  indicated  by  the  broken  lines. 

steadied  by  the  internal  and  external  recti,  the  superior  rectus 
assisting  in  the  elevation  of  the  lid. 

2.  Depression. — Looking  straight  downwards  is  produced  by 
the  inferior  rectus  and  the  superior  oblique,  steadied  by  the 
lateral  recti,  the  inferior  rectus  assisting  in  the  depression  of  the 
lower  lid. 

3.  Abduction. — The  eye  is  turned  straight;^ out  by  the  external 


CONVERGENCE 


41 


rectus,  assisted  at  the  extremity  of  its  action  by  the  superior 
and  inferior  recti. 

4.  Adduction. — The  eye  is  turned  straight  in  by  the  internal 
rectus,  assisted  at  the  extremity  of  its  action  by  the  superior  and 
inferior  recti. 

When  both  eyes  look  to  the  right,  we  have  contraction  of  the 


Fig.  29. — Diagram  of  the  Connections  of  the  Nuclei  of  the 
Lateral  Recti  Muscles.     (After  Ross.) 

C  C,  Cortex  of  right  and  left  cerebral  hemispheres;  i,  2,  fibres 
of  the  pyramidal  tract  uniting  C,  the  cortex  of  the  right 
hemisphere,  and  r'  and  er',  the  nuclei  of  the  left  internal 
and  external  rectus;  i',  2',  fibres  of  the  pyramidal  tract 
connecting  the  cortex  of  the  left  hemisphere  with  r  and  er, 
the  nuclei  of  the  right  internal  and  external  rectus  muscles ;  c, 
fibres  of  the  corpus  callosum  uniting  identical  regions  of 
the  two  hemispheres;  c',  commissural  fibres  connecting  the 
spinal  nucleus  of  the  internal  rectus  of  one  eye  with  that  of 
the  external  rectus  of  the  opposite  eye ;  c",  suggested  com- 
missural fibres  connecting  the  nuclei  of  the  two  internal  recti. 


right  external  and  left  internal  recti,  and  when  they  look  to  the 
left,  contraction  of  the  left  external  and  right  internal  recti. 

Movement  of  the  eyes  up  and  in  is  produced  by  i  and  4 — viz., 
superior  rectus,  inferior  oblique,  and  internal  rectus,  movement 
down  and  out  by  2  and  3,  and  so  on. 


42  THE    REFRACTION   OF   THE   EYE 

The  external  rectus  is  supplied  by  the  sixth  nerve,  the  superior 
oblique  by  the  fourth,  and  the  others  by  the  third. 

Convergence  of  the  eyes  is  produced  by  the  associated  move- 
ments of  both  the  internal  recti.  The  nuclei  {r  r',  Fig.  29)  of 
that  part  of  the  third  nerve  which  supplies  these  muscles  may  be 
connected  by  fibres  {c"),  illustrating  the  principle  that  there  is 
bilateral  association  of  the  nerve  nuclei  of  muscles  bilaterally 
associated  in  their  action  (Broadbent).  This  explains  the  con- 
vergence of  a  covered  eye.  A.  Graefe  says  that  one  of  the  factors 
causing  the  covered  eye  to  converge  is  a  "  Convergenzgefiihl," 
or,  as  Hansen  Grut  expresses  it,  a  "  Nahebewusstsein  " — a  con- 
sciousness of  nearness.  Landolt  denies  this,  and  asserts  that  the 
excluded  eye  fixes  correctly  through  the  connection  between 
accommodation  and  convergence  alone. 

It  is  important  to  remember  that  when  a  stimulus  passes  pri- 
marily to  the  nucleus  of  the  internal  rectus,  it  is  associated  with 
the  same  muscle  of  the  opposite  side,  and  convergence  takes 
place;  whereas  the  conjugate  movements  of  the  eyes  to  the  right 
or  left  are  produced  by  stimuli  passing  primarily  to  the  nucleus 
of  the  external  rectus,  which  nucleus  is  connected  with  the  nucleus 
of  the  internal  rectus  of  the  opposite  side  (Fig.  29).  We  may 
have  both  these  stimuli  occurring  at  the  same  time — viz.,  primary 
stimulus  to  the  internal  recti  to  converge,  and  to  the  external 
rectus  of  one  side  associated  with  the  internal  rectus  of  the  other 
side — to  produce  lateral  movements  of  the  eyes. 

The  oculo-motor  centre  (Fig.  30,  o.m.c.)  is  situated  beneath  the 
floor  of  the  aqueduct  of  Sylvius.  It  includes  (i)  the  accommoda- 
tion centre  (a),  lying  most  anteriorly  near  the  middle  line,  and 
(2)  the  pupil  constrictor  centre  (p)  .  The  nucleus  of  the  internal 
rectus  (i.R.)  lies  further  back.  Filaments  pass  along  the  third  or 
oculo-motor  nerve  from  these  centres  to  the  ciliary  muscle,  the 
sphincter  of  the  iris  and  the  internal  rectus,  and  are  so  associated 
that  contraction  of  the  ciliary  muscles  for  accommodation,  of  the 
pupils,  and  of  the  internal  recti  for  convergence,  are  all  three 
associated  in  their  actions.  One  impulse — viz.,  a  psychical 
impression,  a  wish  to  look  at  a  near  object — passes  from  the 
motor  centre  in  the  cortex  of  the  brain  to  these  nuclei,  and  the 
result  of  this  one  impulse  is  the  united  action  of  these  different 
muscles;  the  action  is  not  always  simultaneous,  for  convergence 
often  lags  behind  accommodation  (see  page  56). 

Many  people  can  voluntarily  squint  inwards,  but  they  will  be 
found  to  accommodate  for  a  near  point  at  the  same  time;  some 
few  can,  however,  do  so  without  accommodating,  and  in  such  cases 
the  psychical  impression  probably  passes  straight  to  the  nucleus 
of  the  rectus  internus  by  vs'  (Fig.  30). 

Binocular  Vision. — Man  has  binocular  vision — that  is, 
the  image  from  an  object  falls  upon  the  retina  of  each 
eye  simultaneously,  and  in  normal  binocular  vision  on 
exactly  the  same  region  of  the  retina;  for  if  the  images 


BINOCULAR  VISION 


43 


did  not  overlap,  two  images  would  be  seen,  and  so-called 
"  double  vision  "  would  be  the  result.  The  absence  of 
double  vision  does  not  necessarily  imply  the  presence  of 
normal  binocular  vision  with  fusion  of  the  two  images, 
for  one  eye  may  be  blind  or  its  image  suppressed  by  the 
brain  (monocular  vision).  Many  people  use  one  eye 
only,  for  years,  without  discovering  the  fault.  The  best 
and  quickest  test  for  determining  whether  binocular 


Fig.  30. — Scheme   showing  the  Oculo-Motor   Centre   and 
Some  of  its  Connections.     (Adapted  from  Erb.) 

P5,  Psychical  impression  (the  wish  to  accommodate  being  the 
stimulus) ;  ps',  psychical  impression  for  voluntary  converg- 
ing strabismus;  a,  accommodation  centre  with  motor  nerve 
to  ciliary  muscle,  and  p,  centre  for  the  sphincter  of  the  iris 
with  motor  nerve,  the  two  forming  the  oculo -motor  centre, 
o.M.c;  I.R.,  internal  rectus  centre,  with  motor  nerve  to 
internal  rectus  muscle;  o.n.,  optic  nerve  from  retina  to  o.c, 
optic  centre,  and  connected  with  p,  the  papillary  centre; 
X  is  the  seat  of  the  lesion  causing  reflex  pupillary  immobility. 


vision  is  present  or  not,  is  Snellen's  apparatus,  described 
on  page  153.* 

Whereas,  then,  in  discussing  accommodation  we  con- 
sidered the  eye  simply  as  an  optical  apparatus,  now  we 

*  Any  of  th3  tests  for  latent  deviation,  mentioned  later,  may 
also  be  employed. 


44  THE  REFRACTION   OF  THE    EYE 

must  consider  the  two  eyes  together  as  forming  one  whole, 
and  on  their  proper  associated  movements  must  depend 
perfect  binocular  vision. 

If  binocular  vision  be  impossible,  through  some  great 
defect  of  the  optical  apparatus  or  the  muscles,  no  attempt 
will  be  made  to  produce  it,  and  no  strain  will  follow.  On 
the  other  hand,  apparently  normal  binocular  vision  may 
exist;  but  to  produce  this,  a  demand  in  excess  of  the 
power  is  put  upon  a  muscle  or  a  set  of  muscles,  and  the 
result  is  strain,  either  producing  or  tending  to  produce 
the  symptoms  of  muscle  strain. 

The  Relation  of  the  Two  Eyes  to  Each  Other  in 
Normal  Distant  Vision. — Michael  Foster  says  that  the 
primary  position  of  the  eyes  is  "  that  which  is  assumed 
when,  with  the  head  erect  and  vertical,  we  look  straight 
forwards  to  the  distant  horizon;  the  visual  axes  of  the 
two  eyes  are  then  parallel  to  each  other  and  to  the 
median  plane " — that  is,  in  ideal  binocular  distant 
vision,  the  eyes  being  at  rest  and  all  the  muscles  in 
equilibrium  with  respect  to  each  other,  the  visual  axes 
are  parallel. 

Test  for  Latent  Deviation  of  the  Eyes  for  Distance. — If  a 
person  with  normal  vision  be  directed  to  look  at  an  object 
in  the  distance,  and  one  eye  be  covered  for  twenty  or 
thirty  seconds,  if  there  be  any  latent  deviation  it  becomes 
as  a  rule  manifest,  and  on  uncovering  the  eye  there  will 
be  diplopia  for  a  brief  space  of  time,  the  covered  eye 
moving  (in  order  to  fuse  the  two  images) — in,  if  there  be 
latent  divergence,  and  out,  if  convergence.  A  more 
accurate  method  of  conducting  this  test  is  to  destroy  the 
possibility  of  binocular  vision — i.e.,  fusion — by  means  of 
a  prism,  with  its  base  up,  placed  before  one  eye,  or,  better 
still,  by  the  apparatus  suggested  by'^Maddox,  called  the 
"  glass  rod  test  ";  by  which  means  we  can  not  only  at 
once  detect  concealed  deviation,  but  can  also  measure 
the  amount. 

The  Maddox  Test. — A  glass  rod  (Fig.  31,  a)  is  arranged 


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MADDOX  ROD   TEST  47 

in  a  metal  disc,  which  fits  into  the  trial  frame.*  If  this 
rod  be  placed  before  one  eye,  the  other  eye  remaining 
uncovered,  and  a  small  flame  be  looked  at  from  a  distance 
of  more  than  4  metres,  the  eye  in  front  of  which  the  rod 
is  placed  sees  the  flame  merely  as  a  streak  of  light,  and, 
the  images  of  the  tv/o  eyes  being  so  dissimilar,  there  is 
no  desire  on  the  part  of  the  brain  to  fuse  them;  conse- 
quently the  two  eyes  assume  their  position  of  rest.  If 
the  rod  be  placed  horizontally  in  front  of  the  right  eye, 
there  is  a  vertical  streak  of  light,  and  if  this  streak 
coincide  with  the  image  of  the  candle  seen  by  the  left 
eye,  the  visual  axes  are  parallel  (orthophoria) ;  but  if  it 


Fig.  31. 

do  not,  then  when  the  streak  is  on  the  same  side  as  the 
rod  (in  this  case  the  right  side)  there  is  latent  convergence 
(homonymous  diplopia),  when  on  the  other  side  there  is 
latent  divergence  (crossed  diplopia).  If  a  scale  be  used 
as  suggested  by  Maddox  (see  plate),  the  number  on  the 
scale  through  which  the  streak  of  light  passes  records 
the  amount  of  diplopia ;  or  prisms  may  be  put  up  in  front 
of  the  other  eye,  or  the  rotary  prism  used  (see  page  7). 
The  weakest  prism  that  causes  the  two  images  to  coin- 
cide records  the  amount  of  diplopia.     The  Maddox 

*  Fig.  31,  b,  represents  a  simple  form  of  this  apparatus  which 
can  be  made  by  uniting  four  or  five  glass  rods  with  seaUng-wax. 
This  must  be  held  before  the  eye,  as  it  does  not  fit  into  a  trial 
frame. 


4^  THE  REFRACTION  OE  THE  EYE 

distance  scale  is  marked  for  5  metres,  and  roughly  every 
3j°  represents  a  metre  angle.  To  be  quite  exact,  every 
3°  40'  or  32  cms.  is  a  metre  angle.  If  the  scale  be  used 
at  4  metres,  then  every  25*5  cms.  represents  a  metre 
angle. 

If  we  wish  to  measure  vertical  deviations,  we  turn  the 
rod  vertically,  and  thus  obtain  a  horizontal  streak  of 
light.  If  this  streak  pass  through  the  middle  of  the 
flame  there  is  no  vertical  deviation,  but  if  it  be  above  or 
below  there  is  hyperphoria  of  that  eye  which  sees  the 
lower  image — i.e.,  if  the  streak  of  light  be  lower  there  is 
a  tendency  to  upward  deviation  (latent  hyperphoria)  of 
the  eye  in  front  of  which  the  rod  is  placed.  To  measure 
the  vertical  deviation  we  must  use  a  scale,  similar  to 
that  in  the  Plate,  placed  vertically. 

Before  this  test  is  applied  any  refractive  defect  must 
be  corrected.  By  making  a  large  number  of  examina- 
tions by  this  method,  we  can  easily  prove  the  correctness 
of  the  statement,  ihs^.,  for  all  practical  purposes,  the  visual 
axes  of  the  two  eyes  in  normal  binocular  vision  are  parallel. 
So  much  for  what  is  called  the  "  static  equilibrium  "  of 
the  ocular  muscles.  Now  we  proceed  to  examine  the 
dynamic  condition — that  is,  the  relation  of  the  muscles 
in  binocular  near  vision;  in  other  words,  during  con- 
vergence. 

Convergence  "  is  the  direction  that  the  eyes  must  give 
to  their  lines  of  fixation  in  order  that  they  may  be 
simultaneously  directed  toward  the  point  of  fixation." 
When  both  eyes  are  fixing  an  object  6  metres  (or  more) 
distant,  they  are  parallel,  and  C  (which  represents  con- 
vergence) =  o;  when  the  eyes  simultaneously  fix  an 
object  I  metre  off  in  the  median  line,  both  internal  recti 
contract  and  the  eyes  converge ;  convergence  is  then  said 
to  be  I  metre  angle,  C  =  i  m.a.  This  metre  angle  is  the 
unit  of  convergence.  If  the  eyes  converge  to  a  point 
50  cms.  off,  then  C  =  -Vxr  =  2  m.a.;  if  20  cms.  off, 
C  =  ^■^^-  =  5    m.a.;  and  if  the  object  be  3  metres  off. 


AMPLITUDE  OF  CONVERGENCE  49 

C  =  i  =  *33  m.a.  In  Fig.  32,  E  e  is  the  base  line  con- 
necting the  two  eyes,  and  E  r'  and  e  r'  are  two  lines  at 
right  angles  to  this  base,  and  therefore  parallel.  If  the 
two  eyes  look  at  a  point  R,  the  angle  r'  e  r  is  the  "  metre 


Fig.  32.    (After  Nagel  and  Landolt.) 

angle,"  or,  better  still,  as  r'  e  R  is  equal  to  E  R  p,  the 
latter  may  be  called  the  "  metre  angle."  To  Nagel 
belongs  the  credit  of  devising  this  method  of  measuring 
the  amount  of  convergence.  The  metre  angle  (or  "  Meter- 

4 


50  THE   REFRACTION    OF   THE   EYE 

winkel,"  as  he  calls  it)  of  convergence  corresponds  to 
the  dioptre  of  accommodation.  Thus,  an  emmetfope 
who  is  fixing  binocularly  a  point  i  metre  off  is  using 
I  dioptre  of  accommodation,  and  convergence  is  i  m.a. ; 
and  if  the  point  be  25  cms.  off,  he  is  using  4  dioptres 
of  accommodation,  and  his  amount  of  convergence  is 
-W-  =  4  metre  angles,  and  so  on. 

Amplitude  of  Convergence. — ^We  again  use  Bonders' 
formula,  and,  expressing  the  equation  in  metre  angles, 

c  a  ==  c  p  —  c  r, 

where  "c  a"  represents  the  amplitude,  "c  p"  the 
maximum,  and  "  c  r  "  the  minimum,  of  convergence. 

When  R  is  at  finite  distance  (Fig.  32),  we  have 
ca^cp-  cr;  that  is,  the  amplitude  of  convergence 
is  the  amount  of  convergence  required  to  direct  the 
visual  axes  of  the  two  eyes  simultaneously  to  the  point 
P,  starting  from  the  binocular  distant  point  R.  When 
the  visual  axes  are  parallel,  "  r  "  can  be  ignored,  and 
the  equation  stands — 

c  a  =  c  p. 

When  the  visual  axes  diverge,  E  r",  e  r",  the  axes  will,  if 
prolonged  backwards,  meet  at  a  point  -  R,  which  is 
negative;  the  equation  will  then  be — 

ca  =  cp-  (-cr)  =  cp+cr. 

We  distinguish  the  equation  from  that  used  in  accommodation 
by  prefixing  or  affixing  a  "c,"  thus  : 


ca=:cp-cr, 
a":  =  pc-rc. 


The  Punctum  Remotum  of  Convergence. — Just  as  the 
punctum  remotum  of  accommodation  is  the  expression  of 
the  refraction  of  the  eye  when  completely  at  rest,  so  the 
punctum  remotum  of  convergence  is  the  expression  of  the 


THE   NEAR   POINT  51 

position  of  the  eyes  when  at  rest — that  is,  when  the  impulse 
to  fusion  brought  about  by  binocular  vision  is  removed — so 
that  to  find  R  we  must  find  the  latent  position  of  the  eyes 
for  distance.  This  we  do  by  the  Maddox  test,  and  the 
number  of  metre  angles  read  off  on  the  scale  gives  us 
"  c  r."  When  there  is  no  latent  deviation  "  c  r  "  =  o, 
when  there  is  latent  divergence  "  c  r  "  is  negative,  and 
when  latent  convergence  it  is  positive. 

To  find  "  c  p,"  the  maximum  of  convergence,  we 
direct  the  person  to  fix  binocularly  a  small  test  object 
held,  say,  J  metre  from  the  eyes,  equidistant  between 
them  and  on  the  horizontal  plane  of  the  eyes.  This  may 
be  a  fine  hair  or  wire  stretched  vertically  in  a  frame,  or 
it  may  be  a  luminous  slit,  as  in  Landolt's  ophthalmo- 
dynamometer (Fig.  33);  when  the  object  is  approached 
to  such  a  distance  that  the  test  line  appears  double,  we 
measure  off  the  distance  in  centimetres,  and  divide  this 
into  100,  which  gives  us  the  number  of  metre  angles 
that  "  p  "  is  equal  to.  Suppose  "  c  P  "  to  be  10  cms., 
"  c  p  "  =  "W  =  10  m.a.,  and  if  c  R  be  at  "  infinity," 

a  =  10; 

but  if  there  be  latent  divergence,  say  of   i  m.a.,  r  = 
—  I  m.a.,  and 

a  =  10  -  (-  i)  =  10  +  I  =  II  m.a. 

In  this  test  we  must  be  careful  to  distinguish  between 
mere  haziness  of  the  test  object,  which  is  the  result  of  its 
being  within  the  patient's  accommodation  near  point, 
and  doubling  of  it,  because  the  near  point  of  convergence 
is  often  nearer  than  that  of  accommodation.  We  should, 
therefore,  always  first  ascertain  the  accommodation  near 
point  in  each  eye. 

It  is  generally  considered  that  the  normal  amplitude  of 
convergence  is  10*5  m.a.,  although  it  may  be  15  or  even 
17  m.a. 


52  THE   REFRACTION   OF  THE   EYE 

The  Relative  Range  of  Accommodation  and  Convergence.— 

If  the  latent  position  of  the  eyes  be  tested,  not  only  during  the 
fixation  of  distant  objects  and  of  objects  at  a  reading  distance, 
but  also  for  intermediate  distances  of  fixation,  it  will  be  found 
that,  as  a  rule,  there  is  quite  a  gradual  lagging  of  the  non-fixing 


Fig.  33. — Landolt's  Ophthalmo-Dynamometer. 

This  apparatus  rests  on  a  candle,  which,  when  lighted,  causes  the 
slit  in  the  cylinder  to  appear  as  a  luminous  line. 

eye  behind  the  fixing  one :  a  gradual  increase  of  latent  divergence. 
This  divergence  is  greater  in  myopia  and  less  in  hyperopia  than 
in  emmetropia.  Fig.  34  represents  the  average  curve  of  relative 
latent  deviation  in  emmetropia.  According  to  this  figure,  we 
see  that  with  parallelism,  or  a  condition  almost  approaching  to 


CONVERGENCE 


53 


parallelism  for  distance,  there  is  ^  metre  angle  of  divergence  on 
accommodating  for  ^  metre,  and  a  whole  metre  angle  for  ^  metre 
accommodation — that  is,  that  whereas,  with  both  eyes  fixing,  on 
accommodating  for  J  metre,  4  d  of  accommodation  is  used,  and 
both  eyes  converge  to  a  point  using  4  m.a.  of  convergence, 
when  the  possibility  of  fusion  is  removed  both  eyes  only  con- 
verge to  a  point  ^  metre  off,  using  3  m.a.  of  convergence. 

This  is  no  proof  of  the  existence  of  "  insufficiency  "  of  con- 
vergence ;  all  it  shows  is  that  the  intimate  relation  between  accom- 
modation and  convergence  is  not  absolute. 

All  the  more,  then,  should  we  expect  to  get  latent  divergence 
for  near  points  when  there  is  initial  latent  divergence  for  distance. 
When  there  is  initial  latent  divergence  for  distance,  the  "  lagging  " 
of  the  convergence  behind  the  accommodation  for  near  points  is 


-OC9,           ID               22) 

3D            4'JO 

m 

^^        V 

-■is 

?,m 

\ 

N 

K 

s 

N 
N 
S 

.Ut 

\ 

N 

\ 

4-M 

i-J 

\ 

Fig.  34.     (After  Berry.) 


more  marked  than  when  the  position  of  the  eyes  is  parallelism, 
and  this  produces  a  "  convergence  insufficiency."  We  can 
ascertain  the  presence  of  latent  deviation  in  near  vision  by  the 
Maddox  test.  A  scale  (see  plate,  page  45)  is  held  \  metre  from 
the  eyes,  and  a  prism  of  12°,  base  up,  is  held  before  the  right  eye. 
The  scale  consists  of  a  horizontal  line  with  fine  print  below  it, 
in  the  centre  of  which  is  an  arrow  pointing  upwards.  The  line 
is  divided  in  degrees  which  are  marked  by  figures,  black  on  the 
right  of  the  arrow,  red  on  the  left.  Every  3^°  from  the  arrow 
is  marked  by  a  small  cross  representing  i  m.a.  The  prism 
causes  two  lines  and  two  arrows  to  be  seen,  and  the  patient  is 
instructed  to  fix  the  upper  arrow,  or,  better,  the  fine  print  just 
below  it.  When  there  is  no  latent  deviation  the  two  arrows  are 
in  a  vertical  line.  When  the  lower  arrow  points  to  the  left  (red 
side)  of  the  upper  arrow  there  is  latent  divergence,  and  when  it 


54  THE  REFRACTION   OF  THE   EYE 

points  to  the  right  (black  side)  there  is  latent  convergence  for 
^  metre,  the  amount  of  deviation  being  read  otf  on  the  scale. 
Graefe's  "  dot  and  line  "  test  is  inferior  to  the  foregoing,  as  it 
does  not  record  the  amount  of  the  defect. 

Maddox  maintains  as  a  result  of  his  experiments  that  in  near 
binocular  vision  there  is  always  relative  divergence — that  is,  con- 
vergence always  lags  behind  accommodation.  This  convergence 
is  composed  of  three  factors:  (i)  "  initial  convergence  "  (this,  of 
course,  exists  only  when  there  is  latent  convergence)  due  to 
the  relaxation  of  the  external  recti  which  are  maintaining  paral- 
lelism {p  p,  Fig.  35),  and  the  eyes  assuming  their  position  of  rest 
i  i  ;  (2)  accommodative  convergence — i.e.,  the  amount  of  con- 
vergence which  is  called  forth  by  the  accommodative  effort 
which  brings  the  axes  to  a  a  ;  and,  lastly,  (3)  the  "  fusion  supple- 
ment," which  is  the  result  of  the  desire  for  single  vision,  and 
brings  the  axes  to   0.     This  "  fusion  supplement  "  is   demon- 


FiG.  35.     (Maddox.) 

strated  by  holding  a  pen  midway  before  the  eyes  of  a  patient  at 
the  distance  of  the  convergence  near  point,  and  telling  him  to  fix 
the  tip  of  the  pen;  if  now  one  eye  is  covered,  this  covered  eye 
will  markedly  turn  out,  and,  on  uncovering,  the  patient  will  for 
a  moment  have  diplopia,  the  eye  making  an  incursion  to  recover 
binocular  vision.  The  amount  of  the  excursion  on  covering,  or 
incursion  on  uncovering,  represents  the  fusion  supplement  which 
the  demand  for  binocular  vision  calls  forth.  This  experiment 
can  be  made  on  most  people,  and  is  no  proof  of  "  insufficiency  " 
of  convergence. 

Although  accommodation  and  convergence  are  intimately 
connected,  this  connection  is  not  absolute.  We  can  prove  this 
experimentally  by  altering  our  accommodation  without  changing 
our  convergence,  as  in  looking  at  an  object  with  both  eyes  before 
which  we  place  weak  convex  and  concave  glasses,  and  also  by 
altering  our  convergence  without  changing  our  accommodation 


CONVERGENCE 


55 


by  placing  before  the  eyes  weak  prisms,  base  in  or  out.  The 
amount  of  dissociation  between  the  accommodative  and  con- 
vergence efforts  is  limited,  and  varies  with  and  in  the  individual ; 
it  can  be  increased  by  practice,  and  it  differs  for  varying  degrees 
of  accommodation  and  convergence.  Fig.  36  shows  the  relative 
amount  of  accommodation  that  can  be  used  with  different  degrees 
of  convergence  in  an  emmetrope  aged  15. 

The  horizontal  figures  record  the  degrees  of  convergence  in 
metre  angles,  and  the  vertical  figures  record  the  degrees  of 
accommodation  in  dioptres.     The  diagonal  d  d  represents  the 


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f8 
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a 

7 

6 

S 
4- 
5 

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/     2    5    4-6    6    7    8    3    fO  /t    tZ  /5  M$-  /S  ^  /7  m  /9  2(i 

Fig.  36. 


convergence,  starting  from  zero — i.e.,  "  infinity  " — and  stopping 
at  5  cms.  (20  metre  angles).  The  vertical  divisions  between  the 
upper  curved  line  p  p  and  the  diagonal,  represent  the  amount  of 
maximum  or  positive  part  of  accommodation,  ascertained  by  the 
strongest  concave  glass  that  can  be  borne  without  prejudice  to 
binocular  and  distinct  vision,  for  any  given  point  of  convergence, 
and  those  between  the  diagonal  and  the  lower  curved  line  y  v 
represent  the  amount  of  minimum  or  negative  part  of  accommo- 
dation, ascertained  by  the  strongest  convex  glass.  Thus,  take 
convergence  for  6  m.a.:  above  we  have  2*5  dioptres  of  positive 
accommodation,  and  below  3  of  negative  accommodation — that 


56  THE   REFRACTION   OF  THE  EYE 

is,  the  relative  amplitude  of  accommodation  for  6  m.a.  of  con- 
vergence is  5*5  in  this  individual.  It  will  be  seen  that  when  the 
convergence  has  reached  lo  m.a.  the  whole  of  the  range  of  accom- 
modation is  negative. 

Accommodation  remaining  fixed,  we  can  estimate  the  amount 
of  relative  convergence  by  means  of  prisms ;  the  strongest  prism, 
base  out,  that  can  be  borne  with  fusion  represents  the  positive, 
and  base  in,  the  negative  part,  of  the  amplitude  of  convergence, 
and,  as  Landolt  has  pointed  out,  we  find  that  Fig.  36  can  be  made 
use  of  to  represent  this.  The  diagonal  d  d  represents  the  accom- 
modation starting  with  eyes  adapted  for  infinite  distance;  the 
positive  portion  of  the  relative  range  of  convergence  is  on  the 
right  of  the  diagonal,  and  is  represented  by  the  horizontal 
divisions  between  d  d  and  r  r,  and  the  negative  portion  is  on  the 
left.  Thus  for  accommodation  at  25  cms. — i.e.,  4  dioptres — we 
see  that  we  have  3  m.a.  on  the  right  and  3 '5  m.a.  on  the  left — that 
is,  while  maintaining  the  same  amount  of  accommodation, 
an  adducting  prism  producing  a  deviation  of  3  m.a.,  and  an 
abducting  prism  requiring  a  diminution  of  3-5  m.a.,  can  be  over- 
come by  the  eyes.  Thus  for  4  dioptres  of  accommodative  power 
in  this  individual,  an  amplitude  of  convergence  of  6-5  m.a.  exists. 
It  is  fortunate  for  the  ametrope  that  this  dissociation  between 
accommodation  and  convergence  is  possible.  A  hyperope  of 
3  D  who  fixes  an  object  binocularly  7,^  cms.  off  must  use  an  addi- 
tional 3  of  accommodation — that  is,  he  must  use  6  altogether — 
but  he  will  only  require  to  converge  to  3  m.a.  If  the  association 
between  accommodation  and  convergence  were  absolute,  he 
would  either  have  to  converge  to  6  m.a.,  and  consequently  squint, 
and  thus  lose  binocular  vision,  or  he  could  keep  binocular  vision 
on  the  condition  that  he  did  not  accommodate  for  this  near 
point;  in  other  words,  he  has  the  choice  between  distinct  vision 
and  binocular  vision — he  cannot  have  both.  Many  hyperopes 
dissociate  these  two  efforts,  and  can  by  practice  and  "  nerve 
education  "  accommodate  in  excess  of  their  convergence  (see 
page  90).  The  difference  in  the  power  to  dissociate  these  two 
efforts  is  one  of  the  explanations  of  the  well-known  fact,  that  of 
two  individuals  having  the  same  refractive  defect,  one  will  squint 
and  the  other  not. 

The  same  necessity  for  dissociation  between  convergence  and 
accommodation  occurs  in  myopia.  A  myope  of  3  d  can  see  an 
object  33  cms.  off  without  any  accommodation,  but  must  con- 
verge to  the  extent  of  3  m.a.  Thus  he  uses  his  convergence  in 
excess  of  his  accommodation. 

Donders  stated  that  accommodation  can  only  be  maintained 
for  a  distance  when,  in  reference  to  the  negative,  the  positive 
part  of  the  relative  range  of  accommodation  is  tolerably  great, 
and  that  the  relative  range  of  accommodation  in  ametropic  eyes 
is  quite  different  from  that  of  emmetropic  eyes,  but  that  it  tends 
to  approach  the  latter  when  the  correction  of  the  error  has  been 
worn  for  some  time. 


CHAPTER  V 

THE  OPHTHALMOSCOPE 

To  understand  the  action  of  the  ophthalmoscope,  the 
following  facts  connected  with  the  Optics  of  Reflection 
should  be  remembered : 

I.  When  light  falls  on  a  plane  mirror  (Fig.  37,  A  b), 
the  angle  of  incidence  is  equal  to  the  angle  of  reflection. 

The  incident  ray  f  d  makes  with  the  perpendicular 
p  D  an  angle  f  d  p,  and  the  reflected  ray  D  E  also  makes 


Fig.  37. 

an  angle  E  d  p,  and  these  two  angles  are  equal  to  one 
another.  Both  incident  and  reflected  rays  are  in  the 
same  plane,  which  is  perpendicular  to  the  mirror. 

2.  "When  parallel  rays  of  light  (Fig.  38,  A  b  and  c  d) 
fall  on  a  concave  mirror,  they  are  reflected  to  a  focus  (f) 
in  front  of  the  mirror,  and  this  principal  focus  is  midway 

51 


58  THE   REFRACTION   OF  THE   EYE 

between  the  mirror  and  the  centre  of  curvature  of  the 
mirror  (o)  and  on  the  principal  axis. 

3.  Rays  of  light  coming  from  a  point  near  the  mirror, 
but  beyond  its  centre,  as  at  L  (Fig.  38),  come  to  a  focus  (/) 
between  the  centre  and  the  principal  focus,  and  the  two 
points  are  conjugate  foci. 

The  Ophthalmoscope. 

If  by  some  contrivance  we  can  manage  to  send  rays  of 
light  from  a  spot  in  front  of  our  eye  into  another  eye,  we 
shall  get  some  of  those  rays  returning  to  our  eye  after 


Fig.  38. 

being  reflected  from  the  retina  of  the  observed  eye,  if 
the  media  be  clear,  and  the  pupil  of  the  observed  eye, 
instead  of  appearing  black,  will  appear  red. 

This  can  be  done  in  the  simplest  manner  by  a  piece 
of  glass  plate.  If  Ohd  is  the  observed  eye,  and  Ohr 
the  observer's  eye  (Fig.  39),  in  front  of  which  is  inclined 
a  glass  plate  G  l,  the  rays  of  light  passing  from  l  are 
reflected  partly  at  g  l  into  Ohd,  return  along  the  same 
path,  passing  through  the  plate,  and  enter  the  observer's 
eye.  As  only  a  few  rays  find  their  way  to  the  observer's 
eye,  the  light  is  very  feeble.  This  was  the  principle 
of  Helmholtz's  first  ophthalmoscope,  and  he  improved 


THE   OPHTHALMOSCOPE  59 

it  by  placing  together  several  glass  plates,  and  thus 
increasing  the  luminosity.  If  for  these  glass  plates  a 
mirror  with  a  central  hole  is  substituted,  more  rays  still 
will  pass  into  the  eye;  and  these  rays  returning,  more 
will  pass  through  the  hole  in  the  mirror  into  the  ob- 
server's eye,  and  a  brighter  image  of  the  fundus  will  be 
seen.  A  still  greater  improvement  results  if  we  use  a 
concave  mirror,  as  the  light  is  more  concentrated. 

Such  is  the  simple  ophthalmoscope — viz.,  a  mirror 
with  a  central  sight-hole  supported  on  a  handle. 

The  ophthalmoscope  has  been  further  improved  by 
adding  an  arrangement  of  lenses  of  different  strength, 


which  can  be  turned  into  position  in  front  of  the  sight- 
hole,  so  that  if  the  eyes  of  the  observer  or  observed  are 
ametropic,  a  clear  image  of  the  fundus  can  be  obtained 
by  the  correcting  lens. 

The  Qualities  of  a  Good  Refraction  Ophthalmcscope. — 
The  mirror  should  be  concave,  with  a  focus  of  from  14  to 
17  cms.  It  should  be  oblique,  and  capable  of  being 
turned  round  so  that  it  can  be  used  for  either  eye.  This 
obliquity  of  the  mirror  enables  the  observer  to  approach 
very  near  the  observed  eye  without  cutting  off  any  of  the 
light,  and  also  permits  the  correcting  glass,  when  used, 
to  be  in  a  position  parallel  to  the  vertical  plane  of  the  eye. 
When  the  oblique  mirror  is  not  required,  as  in  the  "  in- 
direct "  method  and  in  the  "  shadow  test,"  a  "  straight  " 


6o 


THE  REFRACTION  OF  THE  EYE 


mirror  should  be  substituted  for  it.  This  can  be  done  by 
changing  the  mirrors,  or,  better  still,  by  an  arrangement 
like  the  nosepiece  of  a  microscope,  to  which  both  mirrors 
are  attached,  either  of  them  being  turned  into  position 
as  required.  A  further  improvement  can  be  made  by  the 
"  straight  "  mirror  being  plane  on  one  side  and  concave 
on  the  other,  and  fixed  with  a  spring  hinge,  so  that  either 
side  of  the  mirror  can  be  used  as  desired.  The  mirror 
should  be  perforated;  imperforate  mirrors  (with  a 
central  hole  in  the  silvering)  are  not  so  good,  as  the  glass 
reflects  some  of  the  light  that  should  enter  the  observer's 


Fig.  40. — Morton's  Ophthalmoscope. 

The  rotating  wheel  is  made  to  serve  as  a  pupiliometer,  the  discs 
being  numbered  from  i  to  8  mm. 

eye.  The  aperture  in  the  mirror  should  not  be  too  small, 
otherwise  too  little  light  will  reach  the  eye  of  the  ob- 
server; its  diameter  should  be  about  3  mm.  anteriorly 
(the  glass  side),  and  somewhat  wider  behind,  and  the 
sides  of  the  tube  should  be  well  blackened.  The  cor- 
recting lenses  of  the  ophthalmoscope  should  not  be  too 
small;  they  should  have  a  diameter  of  not  less  than 
5  mm.  There  should  not  be  too  many  of  them,  and 
never  more  than  two  superimposed.  The  best  plan  is  to 
have  the  glasses  ordinarily  used  arranged  round  the  rim 
of  one  disc,  and  those  less  used  arranged,  either  on 
another  disc,  or  on  a  movable  quadrant.     The  number 


THE   OPHTHALMOSCOPE 


6l 


of  ophthalmoscopes  on  the  market  is  large,  but  the  best, 
and  certainly  the  most  popular,  is  Morton's  (Fig.  40). 


Fig.  41. — Electric  Ophthalmoscope. 


The  Electric  Ophthalmoscope  (Fig.  41).— This  instru- 
ment has  completely  revolutionized  direct  ophthalmo- 


62  THE   REFRACTION    OF   THE    EYE 

scopy.  The  management  of  the  Ught,  when  using  the 
old-fashioned  instrument,  has  always  been  a  trouble 
to  the  beginner  or  the  practitioner  who  only  occasionally 
uses  the  instrument;  this  trouble  is  removed  in  the 
electric  ophthalmoscope  because  the  Ught  is  concealed 
in  a  tube  near  the  mirror  and  is  fed  by  a  battery  in  the 
handle.  But  even  for  the  oculist  who  is  an  adept  at 
using  the  old  instrument  the  advantages  of  the  electric 
ophthalmoscope  are  very  marked.  As  the  light  has 
not  to  be  considered,  the  instrument  can  be  brought  so 
near  the  eye  that  it  can  almost  touch  the  cornea,  and 
consequently  a  dark  room  is  not  necessary,  as,  by  turn- 
ing the  patient  with  his  back  to  the  window,  we  can 
easily  examine  the  whole  of  the  fundus,  and,  moreover, 
the  patient  can  be  examined  in  any  position. 

A  further  improvement  is  obtained  by  using  a  Marple 
Mirror.    This    mirror,    instead    of    having    a    central 


Fig.  42. — The  Marple  Mirror. 

opening,  has  a  U-shaped  one  (see  Fig.  42),  with  the 
result  that  there  is  little  or  no  reflex  from  the  centre  of 
the  cornea,  and  it  is  often  quite  possible  to  examine  the 
macula  through  a  pin-point  pupil. 

With  this  instrument  and  a  combined  concave  and 
plane  mirror  (Fig.  50,  page  83),  and  a  focusing  lens,  no 
further  apparatus  is  required,  and  the  old-fashioned 
refraction  ophthalmoscope  can  be  dispensed  with. 

The  convex  lens  or  focus  glass  used  in  the  "  indirect  " 
method  should  have  a  focus  of  about  8  cms. — i.e.,  be 
about  13  D — and  should  have  a  diameter  of  about  6  cms. 
The  lens  usually  supplied  with  ophthalmoscopes  is  much 
too  small.  The  glass  should  be  kept  clean  and  free  from 
scratches. 


THE    OPHTHALMOSCOPE  63 

The  Different  Methods  of  Examining  the  Eye  with 
the  Ophthalmoscope. 

1.  The  indirect  method. 

2.  The  direct  method. 

3.  The  "  shadow  test,"  or  retinoscopy. 

The  patient  should  be  in  a  darkened  room. 

The  light  used  should  be  on  an  adjustable  bracket  if 
possible;  any  kind  of  light  will  do  if  it  has  a  broad, 
steady,  white  flame,  but  the  electric  light  in  a  ground- 
glass  globe  is  the  best,  as  it  gives  off  less  heat. 

Before  commencing  the  ophthalmoscopic  examination, 
the  eye  should  be  thoroughly  examined  by  the  oblique 
or  focal  illumination.  For  this  purpose  put  the  light 
on  a  level  with  the  patient's  eye,  on  the  same  side  as  the 
eye  to  be  examined,  and  about  12-15  inches  from  it, 
and  with  the  focus-glass  throw  a  luminous  spot  on  the 
cornea.  By  moving  the  lens  about,  the  whole  surface 
of  the  cornea,  the  anterior  chamber,  iris,  and  anterior 
surface  of  the  crystalline  lens,  can  be  examined.  This 
examination  is  further  aided  by  viewing  the  illuminated 
spot  through  a  strong  magnifying-glass,  and  one  of  the 
best  is  Voigtlaender's.  This  preliminary  examination 
gives  valuable  information  as  to  the  translucency  of  the 
media,  etc. 

I.  The  Indirect  Method. — Place  the  light  close  to 
the  patient's  head  and  a  little  behind,  so  that  no  light 
can  reach  the  eye  to  be  examined  directly. 

Use  the  "  straight  "  concave  mirror,  holding  it  about 
15  inches  from  the  eye,  thus  lighting  up  the  fundus, 
and  making  the  pupil  appear  red  (if  the  media  are 
transparent),  and  detecting  opacities  of  the  cornea,  lens, 
and  vitreous  (the  latter  are  best  seen  with  a  plane 
mirror  and  faint  light) . 

Still  using  the  same  mirror,  put  up  the  focus-glass, 
holding  it  by  the  left  index-finger  and  thumb,   and 


64 


THE   REFRACTION   OF   THE   EYE 


steadying  it  by  resting  the  remaining  fingers  of  the 
left  hand  on  the  patient's  brow.  By  this  means  an 
inverted  image  of  the  fundus  is  seen.  This  is  called 
the  "  indirect  method." 

The  observer  recognizes  that  the  picture  is  inverted 
by  slightly  moving  his  head  or  the  focus-glass,  and 
finding  that  the  image  moves  in  the  opposite  direction. 

The  manner  in  which  this  inverted  image  is  formed 
is  shown  by  the  following  figures. 


Fig.  43.     (After  Fick.) 


The  focus-glass  held  in  front  of  the  eye  makes  the 
eye  myopic,  and,  according  to  the  refraction  of  the 
eye,  this  inverted  image  will  be  nearer  or  further  from 
the  lens. 

When  the  observed  eye  is  emmetropic,  the  rays 
coming  from  the  eye  (Fig.  43,  e)  are  parallel,  and  focus 
at  the  principal  focus  of  the  focus-glass;  and,  more- 
over, as  the  rays  emerging  from  the  eye  are  parallel,  it 
does  not  matter  where  the  focus-glass  is  placed ;  nearer 


THE   OPHTHALMOSCOPE  65 

or  further  from  the  eye,  the  image  must  necessarily 
always  be  the  same  size. 

In  hyperopia  (Fig.  43,  h)  the  rays  emerging  from  the 
eye  are  divergent,  and,  passing  through  the  focus- 
glass,  they  form  a  larger  image  than  in  emmetropia, 
and  this  image  is  further  from  the  lens  in  front  of  its 
principal  focus;  on  withdrawing  the  lens  from  the 
eye,  the  image  is  formed  on  the  other  side  of  the  lens, 
nearer  and  smaller. 

In  high  hyperopia  the  image  is  so  far  in  front  of  the 
focus-glass  that  the  observer  will  have  either  to  move 
back,  or  to  accommodate,  in  order  to  get  a  distinct 
view  of  the  inverted  image. 

If  with  the  mirror  alone,  still  held  at  some  distance 
from  the  eye,  we  can  recognize  fundus  details  nol  in- 
verted— that  is,  in  their  true  position — we  are  dealing 
with  high  hyperopia. 

In  myopia  (Fig.  43,  m)  the  rays  emerging  from  the 
eye  are  convergent,  and  form  an  inverted  aerial  image 
in  front  of  the  eye,  and  the  focus-glass  shows  this 
image  smaller  than  in  emmetropia,  and  nearer  to  the 
lens — in  fact,  within  its  principal  focus;  on  withdraw- 
ing the  lens  the  inverted  image  becomes  larger. 

In  high  myopia  no  focus-glass  is  required  to  see 
the  fundus,  as  the  rays  proceeding  from  the  eye  are  so 
convergent  that  they  come  to  a  focus  at  the  punctum 
remotum  and  form  an  inverted  image. 

In  astigmatism  the  disc  may  appear  oval,  and  the 
shape  will  alter  as  the  focus-glass  is  withdrawn,  accord- 
ing to  the  refraction  of  the  different  meridians. 

The  advantages  of  the  indirect  method  are — 

1.  The  examiner  is  further  from  the  patient  than  in 
the  direct  method  (a  distinct  advantage  in  dealing  with 
certain  patients). 

2.  A  general  "  bird's-eye  "  view  of  the  fundus  is 
obtained. 

3.  No  correcting  glasses  are  needed  in  the  ophthal-' 


66  THE   REFRACTION    OE  THE   EYE 

moscope;  thus,  a  simple  concave  mirror  with  a  central 
hole  is  sufficient. 

4.  It  is  sometimes  easier  to  see  the  fundus  when  the 
pupil  is  small. 

In  looking  at  the  right  disc,  the  patient  should  be 
directed  to  look  past  the  observer's  right  ear,  for  the 
disc  is  on  the  nasal  side  of  the  posterior  pole  of  the 
eye,  and  on  looking  at  the  left  disc  he  should  look 
past  the  left  ear.  It  is  important  to  remember  that 
the  patient  must  look  with  the  eye  not  being  examined ; 
therefore,  in  examining  the  left  eye  by  this  method, 
take  care  not  to  obscure  the  right  eye  with  the  hand 
that  is  holding  the  focus-glass. 

2.  The  Direct  Method.— As  already  stated  above,  this 
method  is  much  simplified  by  the  use  of  the  electric 
ophthalmoscope.  If  the  old-fashioned  instrument  is 
used  the  light  must  be  brought  quite  close  to  the  patient's 
head  and  slightly  behind,  and  on  the  same  side  as  the 
eye  to  be  examined.  The  observer  sits  (or  stands  in  a 
stooping  position)  close  to  the  patient,  and  on  the  same 
side  as  the  eye  to  be  examined,  using  his  right  eye  for  the 
patient's  right  eye,  and  his  left  for  the  patient's  left. 

Use  the  refraction  ophthalmoscope  (without  the 
focus-glass)  and  the  oblique  concave  mirror.  Hold- 
ing the  ophthalmoscope  a  few  inches  from  the  eye, 
reflect  the  light  on  to  the  eye  and  observe  the  red 
pupillary  reflex  through  the  central  hole  of  the  mirror, 
and  then,  without  allowing  the  light  to  leave  the  eye, 
approach  the  eye  as  near  as  possible;  in  fact,  the  ob- 
server's forehead  ought  to  touch  the  patient's  forehead. 
The  fault  that  most  beginners  make  is  not  getting 
near  enough  to  the  eye.  The  observer  must  not  ac- 
commodate, but  look,  as  if  trying  to  see  through  the 
patient's  head,  into  distance.  If  the  observer  or 
patient  have  an  error  of  refraction,  the  wheel  of  the 
ophthalmoscope  must  be  turned  until  the  suitable 
glass  is  found.    To  see  the  macula,  the  patient  should 


THE  OPHTHALMOSCOPE  b'/ 

be  told  to  look  horizontally,  in  front;  if  the  disc  is  to 
be  examined,  he  should  look  slightly  to  the  nasal  side. 


Fig.  44. — Examination   of   the    Erect  Image   when   the  Eye 

EXAMINED   IS    HyPEROPIC,    EMMETROPIC,    OR    MyOPIC.       (HaAB, 

after  Fick.) 

In  each  figure  three  rays  are  shown  emanating  from  a  luminous 
point  on  the  eye-ground.  In  hyperopia  they  diverge  after 
leaving  the  eye,  in  emmetropia  they  are  parallel,  in  myopia 
they  converge :  /,  the  posterior  focus ;  H,  principal  plane  of 
the  dioptric  system  of  the  examined  eye ;  Be.,  observer.  The 
ophthalmoscope  is  not  shown. 


Only  a  small  portion  of  the  fundus  can  be  seen  at 
one  time,  but  this  portion  is  considerably  magnified 


68  THE   REFRACTION   OF   THE   EYE 

(about  15  diameters),  and  consequently  the  minutest 
details  are  visible. 

By  this  method  the  refraction  of  an  eye  can  be  esti- 
mated, which  as  an  objective  method  has,  of  course,  a 
distinct  advantage. 

The  first  duty  of  the  observer — and  most  beginners 
find  this  very  difficult — is  to  relax  his  accommodation. 
The  person  whose  eye  is  being  examined  must  also 
relax  his  accommodation,  which  can  be  done  by  direct- 
ing him  to  look  at  some  object  5  or  6  metres  off  with 
the  other  eye,  or,  better,  by  paralyzing  the  ciliary 
muscle  with  a  cycloplegic.     If  both  the  observer's  and 


obr 

Fig.  45. 

the  observed  eye  are  emmetropic,  all  the  details  of  the 
fundus  will  be  clearly  seen  (Fig.  44,  B).  We  can  easily 
understand  this,  when  we  remember  that  rays  passing 
from  the  mirror  to  the  back  of  the  eye  that  is  being 
examined,  are  reflected  as  parallel  rays  if  the  eye  be 
not  accommodating  and  be  emmetropic,  and  that 
parallel  rays  must  be  focused  on  the  fundus  of  the 
observing  eye  if  it  also  be  emmetropic,  and  its  accom- 
modation be  relaxed  (Fig.  45).  If,  on  the  other  hand, 
the  observer's  eye  under  these  circumstances  accom- 
modate, the  image,  instead  of  being  sharp,  is  blurred. 
It  is  not  only  necessary  to  observe  these  rules  in  order 
to  get  a  clear  picture  of  the  fundus,  but  it  is  of  para- 


THE   OPHTHALMOSCOPE  69 

mount  importance  if  we  wish  to  estimate  correctly 
,  the  refraction  of  the  eye  we  are  examining.  For  this 
reason  it  is  important  that  the  observer  should  esti- 
mate his  own  refraction,  and,  if  there  be  any  error, 
correct  it. 

If  the  observer  be  myopic,  the  fundus  will  be  indis- 
tinct, just  as  is  the  case  wuth  all  distant  objects,  for 
the  rays  coming  from  the  observed  ieye  are  parallel — 
that  is,  as  if  coming  from  a  distant  object.  In  order, 
therefore,  to  obtain  a  clear  view  of  the  fundus,  the 
myope  must  use  a  concave  glass,  and  the  weakest 
concave  glass  he  can  see  distinctly  with  will  be  the 
measure  of  his  myopia,  if  his  accommodation  be  relaxed . 

A  hyperopic  observer  is  in  a  somewhat  better  posi- 
tion, because  he  can  see  the  fundus  if  he  accommo- 
dates; but  as  he  must  relax  his  accommodation  in 
order  to  estimate  the  refraction  of  the  eye  he  is  exam- 
ining, he  must  first  find  his  own  refractive  defect  and 
correct  it.  Unless  he  had  his  defect  properly  corrected 
in  early  youth,  he  has  become  so  accustomed  to  use  his 
accommodation  that  it  will  be  most  difficult-^almost 
impossible — for  him  to  relax  it,  and  the  probability  is 
that,  although  the  convex  glass  he  uses  corrects  his 
defect,  he  nevertheless  cannot  help  using  some  accom- 
modation, and  will  thus  overcorrect  himself,  render- 
ing himself  myopic.  It  necessarily  follows,  therefore, 
that  it  is  most  difficult  for  a  hyperope  to  estimate  the 
refraction  of  an  eye  correctly  by  this  method.  He 
should  use  some  other  method,  such  as  the  "  shadow 
test,"  which  will  be  explained  later. 

We  have  supposed  up  to  now  that  the  observed  eye 
was  emmetropic.  We  will  proceed  to  examine  the 
conditions  that  exist  when  the  observed  eye  is  myopic 
or  hyperopic. 

Examination  and  Measurement  of  a  Myopic  Eye 
by  the  Direct  Method. — The  retina  of  a  myopic  eye  is 
at  the  conjugate  focus  of  an  object  situated  at  finite 


7&  THE   REFRACTION   OF   THE   EYE 

distance  (see  page  99);  consequently  rays  proceeding 
from  the  retina  of  a  myopic  eye  are  focused  at  the  far 
point  when  the  accommodation  is  relaxed  (Fig.  44,  C). 
As  this  far  point  is  at  finite  distance — in  fact,  near  the 
eye — the  rays  are  convergent;  consequently  they  will 
not  be  focused  on  the  retina  of  an  emmetropic  eye 
unless  they  are  made  parallel  by  using  a  suitable  con- 
cave glass  in  the  ophthalmoscope.  This  is  done  by 
turning  the  wheel  of  the  instrument  and  bringing  con- 
cave glasses  before  the  opening,  and  the  weakest  con- 
cave glass  required  is  the  measure  of  refraction  (if  the 
accommodation  of  both  the  observer's  and  the  ob- 
served eye  is  relaxed). 

The  observer  will,  of  course,  be  able,  by  using  his 
accommodation,  to  see  the  fundus  with  a  stronger 
concave  glass  than  is  required,  but  it  will  not  then 
be  the  measure  of  the  myopia.  If  the  observer  be 
a  myope,  and  his  myopia  be  not  corrected  with  glasses, 
to  ascertain  the  refraction  of  the  observed  eye  he 
must  deduct  from  the  concave  glass  he  requires  the 
amount  of  his  own  myopia.  When,  for  instance,  the 
weakest  concave  he  requires  to  see  clearly  the  retina 
of  the  myopic  eye  is  -5,  and  he  himself  is  -2,  then 
the  observed  eye  is  —  3.  When  he  is  hyperopic,  he 
must  add  the  amount  of  his  hyperopia — i.e.,  when 
he  has  hyperopia  of  2,  and  the  weakest  glass  he  re- 
quires is  -5,  the  amount  of  myopia  in  the  observed 
eye  is  -  7. 

The  Examination  and  Measurement  of  a  Hyperopic 
Eye  by  the  Direct  Method  (Fig.  44,  A). — The  rays  emerg- 
ing from  a  hyperopic  eye  are  divergent  (see  page  85),  and 
as  they  must  be  made  parallel  for  an  emmetropic  ob- 
server if  he  wish  to  see  the  fundus  clearly,  a  convex 
glass,  representing  the  amount  of  hyperopia,  must  be 
turned  into  position.  If  the  patient  have  hyperopia 
of  4,  then  +  4  must  be  used.  The  fundus  could  be 
seen  clearly  without  a  glass,  by  accommodation;  but 


THE   OPHTHALMOSCOPE  7 1 

then,  as  it  would  be  impossible  to  measure  the  amount 
of  accommodation  used,  so  would  it  be  impossible  to 
estimate  the  amount  of  hyperopia  in  the  observed  eye. 

A  hyperope  who  is  examining  a  hyperopic  eye  with 
the  ophthalmoscope  must  deduct  the  amount  of  his 
own  hyperopia  from  the  strongest  lens  he  requires  to 
see  the  fundus  with;  i.e.,  if  he  be  hyperopic  to  the 
extent  of  3,  and  the  strongest  convex  glass  he  can 
clearly  see  the  fundus  with,  is  +6,  the  observed  eye 
has  a  hyperopia  of  3  d. 

A  myope,  on  the  other  hand,  as  he  requires  a  weaker 
correcting  glass,  must  add  in  dioptres  the  amount  of 
the  defect;  thus,  when  he  is  myopic  to  the  extent  of  5, 
and  requires  no  glass  to  see  the  fundus  clearly,  the 
eye  that  is  being  examined  is  hyperopic  to  the  extent 
of  5.  Again,  when  his  myopia  is  3,  and  the  strength 
of  the  convex  glass  he  can  use  is  2,  the  amount  of 
hyperopia  present  in  the  observed  eye  is  5 ;  or  when  he 
has  myopia  of  7,  and  he  cannot  see  the  fundus  clearly 
with  any  glass  less  concave  than  3,  the  amount  of 
hyperopia  present  is  7  +  (  -  3) — i.e.,  4  d. 

To  summarize,  we  may  say  that  if  an  ametrope,  to 
see  clearly  the  fundus  of  an  eye  with  the  ophthalmoscope 
and  to  estimate  correctly  its  refraction,  requires — 

(i)  A  glass  of  the  same  kind  as  his  own  ametropia, 
but  stronger,  he  must  deduct  the  number  of  his  own 
from  that  glass. 

Example. — He  has  a  myopia  of  3,  and  requires  -  5  in  the 
ophthalmoscope,  then  the  error  of  the  observed  eye  is  -  2. 

(2)  A  glass  of  the  same  kind,  but  from  one  to  ten 
dioptres  weaker  than  his  own  ametropia,  then  the  eye 
that  is  being  examined  has  an  ametropia  of  from  one 
to  ten  dioptres  of  the  opposite  kind. 

Example. — He  has  a  myopia  of  6,  and  requires  -  5 ;  the  refrac- 
tive defect  of  the  observed  eye  is  + 1 .  If  he  require  -  4,  it  is 
+  2,  and  so  on.  He  has  hyperopia  +4,  and  requires  +3;  then 
the  refractive  error  of  the  observed  eye  is  -  i , 


72  THE   REFRACTION   OF  THE   EYE 

(3)  A  glass  neither  of  the  same  kind  nor  strength, 
then  the  refraction  of  the  observed  eye  is  the  opposite 
to  that  of  the  observer's,  and  the  amount  is  equal  to 
the  addition  of  the  number  of  dioptres  of  each. 

Example. — He  has  myopia  of  5,  and  requires  +  3 ;  the  refrac- 
tion of  the  observed  eye  is  +  8.  He  has  hyperopia  of  3,  and 
requires  -  2 ;  the  error  is  -  5. 

It  should  be  borne  in  mind  that,  to  insure  the  exact 
measurement  of  the  patient's  refraction  by  means  of 
the  ophthalmoscope,  the  yellow  spot  must  be  looked 
at.  If  the  patient  be  not  under  the  influence  of  a 
mydriatic,  this  is  not  always  easy,  for  not  only  does 
the  pupil  contract  when  the  examined  eye  is  turned 
towards  the  mirror,  but  the  light  reflex  from  the  cornea 
interferes  very  much  with  the  view  unless  the  electric 
ophthalmoscope  with  Marple  mirror  is  used;  and, 
further,  the  absence  of  any  large  structure,  such  as  the 
retinal  vessels,  makes  it  diihcult  to  secure  the  correct 
focus.  As  a  rule,  all  that  is  seen  at  the  macula  is  a 
slight  stippling,  produced  by  the  irregular  deposit  of 
retinal  pigment,  and  when  this  pigment  is  specially 
pronounced  we  get  a  bright  ring  or  crescent  at  the 
fovea.  This  is  the  foveal  reflex;  and  although  it  is 
slightly  in  front  of  the  retina,  the  distance  is  so  small 
that  it  can  be  ignored,  and  this  foveal  reflex  can  be 
focused  and  made  use  of  in  this  manner,  for  ascertaining 
the  refraction.  If  it  cannot  be  used  in  this  way,  through 
being  too  faint,  we  must  focus  a  small  retinal  vessel 
passing  from  the  disc  to  the  macula. 

The  beginner  will  find  that  the  easiest  part  to  focus 
is  the  temporal  side  of  the  disc,  for  its  margin  here  is 
generally  very  well  defined. 

The  Measurement  of  Astigmatism. — It  is  very  diffi- 
cult, if  not  impossible,  to  diagnose  low  errors  of 
astigmatism  by  the  ophthalmoscope,  but  an  error  of 
one  dioptre  or  more  is  revealed  by  portions  of  the 
fundus  picture  being  out  of  focus,  and  by  our  inability 


THE  OPHTHALMOSCOPE  73 

to  get  a  clear  picture  of  all  parts  at  the  same  time  by 
any  of  the  spherical  glasses  in  the  ophthalmoscope. 
Some  ophthalmoscopes  have  cylindrical  glasses  fixed 
in  them,  but  this  is  not  at  all  necessary,  as  the  astigma- 
tism can  be  approximately  estimated  without  much 
difficulty  by  measuring  the  refraction  of  the  meridians 
at  right  angles  to  each  other  in  the  following  manner: 
Focus,  for  instance,  the  vessels  that  pass  in  a  horizontal 
direction  from  the  disc  to  the  macula,  and  note  the 
glass  in  the  ophthalmoscope  (the  weakest  concave  and 
strongest  convex)  that  is  required  to  give  a  clear 
definition;  this  will  give  the  refraction  of  the  meridian 
at  right  angles  to  the  horizontal  one — viz.,  the  ver- 
tical. Then  focus  the  vessels  that  pass  vertically 
upwards  and  downwards  from  the  disc;  this  will  give 
the  refraction  of  the  horizontal  meridian,  and  the 
difference  between  the  two  glasses  (if  any)  is  the  amount 
of  the  astigmatism  (if  any). 

When  the  chief  meridians  are  not  vertical  and  hori- 
zontal, but  oblique,  we  can  then,  say,  focus  the  vessels 
passing  upwards  and  outwards  from  the  disc,  and 
when  we  have  focused  these  vessels,  if  astigmatism 
exist,  the  vessels  passing  downwards  and  outwards  will 
not  be  in  focus,  but  will  be  either  blurred  or  invisible, 
and  we  proceed  to  find  the  glass  that  is  necessary  to 
bring  these  latter  vessels  into  focus,  and  so  on.  If 
the  correcting  glass  be  a  large  one,  we  must  be  care- 
ful to  look  through  the  centre,  for  if  we  look  through 
the  glass  obliquely  we  shall  get  an  appearance  as  if 
produced  by  astigmatism,  which  might  not  be  present. 

In  estimating  the  refraction  by  means  of  the  ophthalmoscope, 
as  above  explained,  the  observer  shoald  approximate  his  eye  as 
much  as  possible  to  the  eye  that  is  being  examined,  as  the  value 
of  the  lens  is  altered  by  altering  the  distance;  a  concave  glass  is 
weakened  and  a  convex  glass  strengthened  by  removal  from 
the  eye.  It  is  for  this  reaso.i  that  old  people  are  often  seen  to 
wear  their  glasses  low  down  o.i  the  nose,  the  strength  of  the 
CO  a  vex  glass  being  slightly  increased.  This,  of  coarse,  specially 
refers  to  lenses  of  high  po.ver;  therefore,  the  further  away  we 


74  THE   REFRACTION   OF   THE   EYE 

hold  the  ophthalmoscope  the  more  shall  we  overcorrect  in 
myopia  and  undercorrect  in  hyperopia — i.e.,  the  myopia  of  the 
eye  being  examined  will  be  less,  and  the  hyperopia  more,  than 
that  represented  by  the  ophthalmoscope  glass. 

3.  The  Estimation  of  the  Refraction  by  the  "  Shadow 
Test "  ;  Retinoscopy  ;  Skiascopy. — Seated  at  a  short  dis- 
tance from  the  patient  in  a  dark  room,  if  we  throw  the 
light  on  to  the  patient's  eye  by  means  of  an  ophthal- 
moscopic mirror,  provided  the  pupil  is  normal  and  the 
media  are  clear,  we  observe  the  red  reflex  of  the  fundus ; 
and  if  we  gently  rotate  the  mirror,  the  red  reflex  dis- 
appears, and  darkness  takes  its  place.  The  manner  in 
which  this  darkness  or  shadow  appears  varies  according 
to  the  refraction  of  the  eye. 

We  will  examine  the  behaviour  of  the  shadow  under 
three  conditions : 

1.  When  the  observer  is  beyond  the  patient's  far 
point. 

2.  When  he  is  within  the  patient's  far  point. 

3.  When  he  is  exactly  at  the  patient's  far  point. 

I.  Let  us  suppose  the  surgeon  Ob  (Fig.  46,  A)  examin- 
ing the  patient  Pt  by  this  method,  and  using  the  plane 
mirror,  and  we  will  assume  Pt  to  have  a  refractive  error  of 
over  I  of  myopia.  Ob  is  seated  i  metre  off  Pt,  and  is 
consequently  beyond  P/'s  far  point.  Ob  reflects  the  light 
into  Pt^s  eye  and  observes  the  red  reflex ;  and  if  he  rotate 
the  mirror,  making  the  light  pass,  say,  across  Pfs  face 
from  the  nose  to  the  temple,  he  will  notice  that  the  red 
reflex  disappears,  and  that  darkness  takes  its  place,  and 
in  this  example  the  darkness  or  shadow  comes  over  the 
eye  from  the  temple  towards  the  nose — that  is,  in  the 
opposite  direction  to  the  rotation  of  the  mirror. 

Let  us  see  how  this  has  come  about.  In  Fig.  46,  for 
the  sake  of  clearness,  the  mirror  and  the  light  have  been 
omitted,  and  only  the  rays  proceeding  from  P/'s  fundus 
have  been  drawn. 

All  luminous  rays  proceeding  from  the  fundus  of  Pt 


Fig.  46.     (After  Fick.) 


RETINOSCOPY  77 

through  the  pupil  P  p  (Fig.  46,  A)  either  do  not  reach  the 
eye  Ob,  or  they  impinge  on  Ob's  retina  between  P'  and  p\ 
Thus,  all  rays  from  p,  from  whatever  part  of  Pt's  fundus 
they  come,  must  unite  at  p'  of  Ob  if  they  are  intercepted 
by  06' s  pupil. 

Let  a  he  a,  luminous  point  on  the  fundus  of  Pt  (who  in 
this  case  is  assumed  to  have  a  myopia  of  ov^  i),  then 
at  Pi's  far  point,  somewhere  on  the  line  between  a  and 
the  nodal  point,  an  aerial  image  a'  of  a  will  be  formed. 
Some  of  the  diverging  rays  from  a'  will  reach  Ob,  and, 
passing  through  the  refractive  media,  will  unite  at  a"', 
but  as  Ob's  fundus  intercepts  these  rays,  a  bright 
diffusion  circle  will  be  formed  on  the  upper  part  ot  P'  p' 
{Ob's  fundus),  while  the  lower  part  of  P'  p'  will  be  in 
darkness.  Now,  as  our  retinal  images  are  projected 
inverted,  Ob  sees  the  pupil  of  Pt  light  below  and  dark 
above.  If  the  luminous  spot  a  descend  to  b  in  Pt,  its 
image  ascends  to  b\  and  we  have  a  bright  area  below  in 
P'  p',  and  Ob  sees  in  P^'s  pupil  a  bright  area  passing 
from  below  upwards. 

We  thus  see  how  in  myopia  of  over  i  d,  with  the 
observer  i  metre  from  the  patient,  and  using  a  plane 
mirror,  the  "  shadow  "  moves  against  the  rotation  of  t)  ( 
mirror. 

2.  The  reverse  obtains  when  Ob  is  within  Pt's  far  poin^. 
Let  us  suppose  (Fig.  46,  B)  Pt  to  be  hyperopic.  The 
image  of  a  will  be  at  a',  but  those  rays  that  pass  through 
Ob's  pupil  are  refracted,  and  meet  at  a"  in  front  of  the 
retina,  and,  diverging  again,  meet  Ob's  retina  at  p'  as  a 
diffusion  circle ;  in  this  case  the  bright  area,  being  below, 
is  projected  inversely,  and  Ob  sees  P^'s  pupil  bright 
above  and  dark  below,  and  if  a  moves  down  to  b,  it  will 
be  seen  that  Ob  projects  the  bright  area  moving  down 
also.  Thus,  with  a  plane  mirror,  if  Ob  be  within  Pt's 
far  point,  the  shadow  moves  with  the  mirror;  if  Ob 
be  seated  i  metre  off  Pt,  this  will  occur  in  hyperopia, 
emmetropia,  and  myopia  of  less  than  i  d. 


78  THE  REFRACTION   OF  THE  EYE 

3.  £,astly,  let  us  consider  what  happens  when  Ob  is 
exactly  at  P^'s  far  point,  which,  of  course,  occurs  if  Ob 
be  seated  1  metre  off  Pt,  who  has  a  myopia  of  i  d 
(Fig.  46,  C).  The  illuminated  point  a  has  its  image 
a'  exactly  on  the  pupil  of  Ob,  and  as  the  ray  p  a'  is  re- 
fracted to  p' ,  and  the  ray  P  a'  to  0',  the  entire  area  P'  p' 
is  illuminated,  and  the  entire  pupil  of  Pt  appears  illumi- 
nated to  Ob.  Movement  of  «  to  6  produces  no  effect; 
the  area  P' p'  is  still  illuminated;  but  when  the  luminous 
point  on  P^'s  retina  passes  below  b  or  above  a — that  is, 
outside  the  area  a  b — it  is  focused  on  06's  iris,  and  no 
rays  reach  06's  retina ;  consequently  Ob  sees  the  pupil  Pj^ 
becoming  suddenly  dark,  and  there  is  no  moving  shadow. 
This  point,  when  the  observer's  eye  is  exactly  at  the 
patient's  far  point,  is  called  the  "  point  of  reversal,"  and 
the  whole  principle  of  retinoscopy  is  to  find  this  point. 
In  myopia  Ob  can  move  nearer  to  or  further  from  the 
patient,  and  measure  off  the  distance  of  the  point  of 
reversal,  and  so  obtain  the  refraction  of  that  particular 
meridian ;  but  in  hyperopia  this  cannot  be  done,  so  that 
the  best  method  is  to  work  always  at  one  fixed  point — 
say  I  metre — and  make  the  patient  artificially  myopic, 
if  hyperopia  or  emmetropia  exist,  by  placing  before 
P^'s  eye  convex  glasses ;  if  he  be  myopic,  make  him  less 
myopic  by  using  concave  glasses. 

Let  us  now  examine  a  patient.  The  patient's  eyes 
should  (if  possible)  be  under  the  influence  of  a  cycloplegic, 
which  not  only  gives  us  a  dilated  pupil  and  makes  the 
retinoscopy  easier,  but  insures  the  relaxation  of  the 
ciliary  muscle,  which  of  course  is  essential.  The  patient 
should  be  seated  in  a  dark  room,  with  the  light  above  or 
on  one  side  of  his  head  and  slightly  behind,  so  that  no 
rays  can  reach  the  eye  except  from  the  mirror.  We  pro- 
vide ourselves  with  a  set  of  test  lenses  and  a  trial  frame, 
and  seated,  say,  i  metre  off  the  patient,  we  reflect  the 
light  by  means  of  the  plane  mirror  into  the  patient's  eye, 
"directing  him  to  look  at  the  sight-hole  of  the  mirror. 


RETINOSCOPY  79 

Suppose  we  are.  examining  the  right  eye,  and  rotate  the 
mirror  so  that  the  Hght  passes  across  from  the  patient's 
nose  to  the  temple,  and  suppose  we  notice  that  as  the 
Hght  leaves  the  pupil  a  dark  shadow  takes  its  place, 
passing  across  in  the  same  direction — i.e.,  from  the  nose 
to  the  temple — we  know  from  Fig.  46,  B,  that  we  are 
within  the  patient's  far  point,  and  that  we  are  dealing 
with  a  hyperope  or  emmetrope,  or  myope  of  less  than  i . 
Let  us  place  in  the  trial  frame  +  2 :  we  find,  say,  that  the 
shadow  is  still  moving  with  the  mirror;  we  are  therefore 
dealing  with  hyperopia.  Put  up  +4:  the  shadow  now 
moves  against  the  mirror,  which  means  we  are  outside 
the  patient's  far  point;  put  up  +3,  and  we  find  on 
rotating  the  mirror  that  the  pupil  becomes  suddenly 
dark,  and  there  is  no  shadow  following  with  the  rotation, 
or  passing  against  it.  This,  then,  is  the  point  of  reversal. 
We  have  found  the  point  of  reversal  with  a  +3  lens, 
seated  i  metre  from  the  patient,  which  means  that  this 
meridian  has  a  myopia  of  i  with  a  +3  lens  in  front,  and, 
deducting  i  from  3,  leaves  us  2  as  representing  the 
hyperopia.  If  the  patient  had  been  emmetropic  in  this 
meridian,  a  + 1  lens  would  have  given  us  the  point  of 
reversal  at  i  metre. 

If  the  patient's  eye  have  a  myopia  of  over  i,  when 
we  are  seated  i  metre  off  we  must  be  outside  his  far 
point,  whatever  the  amount  of  myopia,  and  the  shadow 
moves  "  against  "  the  plane  mirror,  and  that  glass  which 
gives  us  the  point  of  reversal  represents  the  amount  of 
myopia  of  that  meridian  with  -  i  added  if  we  are  seated 
I  metre  off  (or  -  2  added  if  seated  50  cms.  off,  or  -  •5 
added  if  2  metres  off) — -that  is,  if  -  5  gives  the  point  of 
reversal,  -  6  is  the  amount  of  myopia.  Some  surgeons 
always  aim  at  reversing  the  shadow — 'that  is,  they 
purposely  go  beyond  the  point  of  reversal,  and  slightly 
overcorrect.  This  is  quite  safe  if  allowance  be  made  fof 
the  overcorrection.  < 

In  the  examples,  we  have  been  ascertaining  the  far 


80  THE   REFRACTION   OF   THE   EYE 

point  of  one  meridian  only — viz.,  the  horizontal;  we 
must  now  proceed  to  examine  the  meridian  at  right 
angles — viz.,  the  vertical — and  if  the  same  glass  give  us 
the  point  of  reversal,  we  know  that  no  astigmatism  is 
present;  but  if  there  be  a  difference,  that  difference 
represents  the  astigmatism.  When  the  astigmatism  is 
great,  and  especially  when  one  meridian  is  emmetropic 
or  made  emmetropic,  the  light  is  seen  to  pass  across  as  a 
bright  band  (Fig.  47),  and  sometimes  two  bright  bands 
are  seen  with  a  dark  band  in  the  centre,  and  as  the 
bright  bands  approximate  each  other,  the  central  dark 
band  disappears,  and  one  bright  band  remains.  This 
has  been  called  the  "  scissor  movement."  , 


Fig.  47. 

In  oblique  astigmatism,  of  course,  the  meridians  are 
not  vertical  and  horizontal,  and  when  the  astigmatism  is 
marked,  the  appearance  of  the  shadow  is  very  character- 
istic, and  a  bright  band  is  seen  passing  obliquely  across 
the  pupil,  although  we  may  be  moving  the  mirror  hori- 
zontally or  vertically  (Fig.  47).  Suppose  we  are  dealing 
with  oblique  mixed  astigmatism,  and,  rotating  the 
mirror  horizontally,  we  observe  a  bright  band  fol- 
lowed by  a  shadow  passing  obliquely  across  the  pupil 
"  with  "  the  plane  mirror,  we  note  the  axis  of  this  bright 
band,  and  also  note  that  the  meridian  is  hyperopic; 
if  we  then  rotate  the  mirror  at  right  angles  to  this  bright 
band,  we  find  that  the  shadow  passes  against  the  move- 


RETINOSCOPY  8l 

ment  of  the  mirror,  showing  that  this  meridian  is  myopic. 
Suppose  the  point  of  reversal  of  the  horizontal  oblique 
meridian  is  obtained  by  -1-3,  and  that  of  the  vertical 
by  -  I,  by  this  we  know  that  the  refraction  of  the  hori- 
zontal meridian  is  +2,  and  that  of  the  vertical  meridian 
is  -  2,  and  there  is  therefore  a  total  astigmatism  of  4. 

The  greater  the  ametropia,  the  nearer  is  the  far  point 
to  the  eye,  and  it  is  of  great  practical  importance  to 
remember  that,  the  greater  the  ametropia,  the  less 
distinct  is  the  shadow  and  the  slower  it  moves;  and  as 
we  approach  the  point  of  reversal  by  using  correcting 
glasses,  we  obtain  an  increasingly  defined  shadow  which 


Fig.  48. 

moves  more  and  more  rapidly.  We  can  thus,  at  once, 
make  a  rough  estimate  of  the  degree  and  kind  of 
ametropia. 

In  Fig.  48,  if  R  be  the  far  point  of  the  myopic  eye,  on 
rotating  the  mirror,  the  shadow  moves  from  R  to  r'  ;  but 
if  the  myopia  be  less,  and  the  far  point  at  M,  the  shadow 
will  have  to  describe  the  larger  arc  M  m'  in  the  same 
time — that  is,  it  will  move  more  quickly. 

In  the  same  way,  if  the  far  point  of  a  hyperope  be  at 
R  (Fig.  49),  the  shadow  will  move  more  slowly  than 
when  the  hyperopia  is  less  and  the  far  point  at  H. 

Some  surgeons  use  the  plane  mirror  at  a  distance  of 
4  metres  always,  and  as  only  •25  d  has  to  be  deducted 
from  the  retinoscopy,  this  small  amount  can  be  ignored, 

6 


82  THE   REFRACTION    OF   THE   EYE 

and  the  point  of  reversal  of  a  meridian  represents  the 
measurement  of  that  meridian. 

When  the  surgeon  is  nearer  than  i  metre,  he  must,  of 
course,  deduct  more  than  i.  For  instance,  suppose  at 
33  cms.  the  point  of  reversal  of  a  meridian  is  obtained 
with  +5,  as  his  far  point  is  J  metre  off,  we  have  made 
the  patient  artificially  myopic  to  the  extent  of  3,  and 
we  must  deduct  this  from  5;  therefore  5-3  (that  is,  2) 
represents  the  hyperopia  of  this  meridian. 

Many  surgeons  still  use  the  concave  mirror.  The 
mirror  should  have  a  focus  of  25  cms.,  and  the  observer 
should  be  seated  a  little  over  a  metre  from  the  patient. 
The  movement  of  the  shadow  is  the  reverse  of  that  which 
takes  place  with  the  plane  mirror — that  is,  the  shadow 


Fig.  49. 

moves  "  with  "  the  mirror  in  myopia  of  over  i,  and 
"  against  "  the  mirror  in  myopia  of  less  than  i,  and  in 
hyperopia  and  emmetropia.  This  is  easily  understood 
if  we  remember  that  with  a  concave  mirror  the  rays  of 
light  converge  to  the  focal  point  of  the  mirror,  and  then 
cross  and  diverge ;  consequently  the  image  thrown  on  a 
screen  by  a  concave  mirror  is  inverted. 

[If  we  reflect  a  lighted  candle  on  to  a  dark  screen  by  a  concave 
mirror  held  further  from  the  screen  than  its  focal  distance,  and 
if  we  then  focus  the  divergent  rays  with  a  convex  lens,  we  shall 
get  an  erect  image,  because  the  rays  have  been  twice  inverted, 
whereas  with  a  plane  mirror  used  in  the  same  manner  we  obtain 
an  inverted  image,  because  there  has  been  only  one  inversion.] 

If  the  observer  be  not  emmetropic,  he  should  wear  his 
correcting  glass.     This,  of  course,  especially  applies  if 


RETINOSCOPY  83 

he  be  myopic.  If  he  be  hyperopic,  he  may  correct  his 
defect  by  accommodation  if  he  choose.  The  point  to  be 
remembered  is,  that  to  practise  retinoscopy  accurately 
the  observer  requires  a  normal  acuity  of  vision.     He 


Fig.  50. 


may  accommodate  as  much  as  he  likes,  as  it  does  not 
affect  the  result. 

As  the  point  of  reversal  is  more  definite  when  the 
shadow  moves  with,  it  is  not  a  bad  plan  to  use  a  plane 


''^■^#;"'^'.  ^  ^^    4r  -^    ^ 


Fig.  51. — Marple's  Skiascopes. 

mirror  when  estimating  hyperopia,  and  a  concave  mirror 
when  estimating  myopia.  These  two  mirrors  can  be 
hinged  together,  and  thus  each  mirror  is  the  handle  and 
cover  of  the  other  (Fig.  50). 

Marpie's  skiascopes   (Fig.   51),   made  by  Meyrowitz  of  New 
York,  are  very  useful,  and  obviate  the  necessity  for  keeping  a 


84  THE   REFRACTION   OF  THE   EYE 

separate  test  case  in  the  dark  room;  they  are  designed  to  be  held 
by  the  patient  before  the  eye  during  retinoscopic  examination. 
Each  contains  a  series  of  six  lenses,  ranging  from  i  to  6  dioptres, 
plus  and  minus  respectively.  In  addition  to  these  lenses  there 
is  on  one  side  a  movable  slide  containing  a  6  d  lens,  which  can  be 
quickly  slipped  up  over  the  other  lenses  one  after  the  other, 
making  further  combinations  from  7  d  to  12  d.  To  determine 
smaller  errors  within  i  d,  a  slide  containing  three  lenses  -25, 
•50,  and  '75  d,  respectively,  is  placed  on  the  other  side,  and 
can  easily  be  brought  before  the  other  lenses.  On  the  skiascope 
containing  the  plus  lenses  the  movable  slide  carries  minus  fraction 
lenses,  and  vice  versa. 


CHAPTER   VI 

HYPEROPIA 

Hyperopia  or  Hypermetropia. — The  hyperopic  eye  is 
the  undeveloped  eye  in  which,  with  accommodation 
at  rest,  parallel  rays  come  to  a  focus  beyond  the  retina 
(Fig.  20,  h),  and  only  convergent  rays  focus  on  the 
retina;  but  as  in  Nature  all  rays  are  either  parallel 
or  divergent,  it  follows  that  the  hyperopic  eye  at  rest 
sees  everything  indistinctly. 

Rays  coming  from  a  point  on  the  retina  diverge, 
and,  on  passing  through  the  dioptric  system,  emerge 
from  the  normal  eye  as  parallel  rays.  In  hyperopia, 
although  they  are  not  so  divergent  as  they  were  before 
refraction,  they  still  diverge  if  the  eye  be  at  rest,  and 
therefore  never  come  to  a  focus  in  front  of  the  eye; 
Jut  when  prolonged  backwards,  they  will  meet  at 
a  point  behind  the  eye — the  punctum  remotum.  This 
punctum  remotum  of  the  hyperope  is  therefore  not 
the  actual  focus  of  the  distant  rays,  but  the  virtual 
focus,   and  is  represented   by  the  negative  sign   -R 

(Fig.  52). 

It  will  be  seen  from  Fig.  52  that  the  more  divergent 
the  rays  are  in  front  of  the  eye,  the  nearer  will  their 
"  backward  prolongation "  focus ;  hence  the  nearer 
-  R  is  to  the  eye,  the  higher  will  be  the  hyperopia. 

This  is  the  same  as  in  myopia — viz.,  the  higher 
the  myopia,  the  nearer  is  r  to  the  eye;  but  the  differ- 
ence is  that  in  myopia  R  is  in  front  of  the  eye,  and 
in  hyperopia  it  is  an  imaginary  point  behind  the  eye. 

«5 


86  THE   REFRACTION   OF  THE   EYE    ^    .      ' 

Thus,  the  degree  of  hyperopia  is  in  inverse  ratio  to 
the  distance  of  the  punctum  remotum.  In  myopia 
this  point  can  be  measured  directly,  but  in  hyperopia 
it  can  only  be  done  indirectly  by  employing  convex 
glasses. 

Suppose  the  punctum  remotum  of  a  hyperope  is 
33  cms.  behind  the  retina.  We  have  seen  that  a  convex 
lens  whose  focal  point  is  33  cms.  is  3  d — that  is,  such  a 
lens  has  the  power  of  converging  parallel  rays  to  a 
point  33  cms.  on  the  other  side  of  the  lens,  and  con- 
versely, rays  diverging  from  a  point  33  cms.  in  front  of 


Fig.  52. 
Showing  the  punctum  remotum  of  a  hyperopic  eye. 

such  a  lens  become  parallel  on  passing  through.  If 
this  lens  be  put  in  front  of  the  eye  of  this  hyperope,  it 
will  so  act  that,  assisted  by  the  dioptric  system  of  the 
eye,  it  will  cause  parallel  rays  to  focus  on  the  retina. 
Hence  the  measurement  of  hyperopia  is  that  convex 
lens  which  enables  the  hyperopic  eye,  at  rest,  to  see 
distinctly  objects  at  a  distance,  and  the  focal  length  of 
such  a  lens  represents  the  distance  of  the  virtual  far 
point  from  the  eye.  In  the  above  example  it  was  found 
that  +3  was  this  lens,  and  we  accordingly  say  that 
this  eye  has  a  hyperopia  of  3. 


HYPEROPIA 


87 


A  hyperope  differs  from  a  myope  in  that  he  can 
correct  his  defect  up  to  a  certain  point;  he  can  by 
accommodation  produce  the  same  effect  on  parallel 
rays  as  if  a  convex  glass  were  placed  in  front  of  the 
eye.  This  apparent  advantage  brings  with  it  many  dis- 
advantages— viz.,  all  the  troubles  incident  to  eyestrain. 

The  hyperopic  eye  is  never  at  rest;  it  has  to  accom- 
modate for  distant  as  well  as  for  near  objects.  The 
emmetrope's  ciliary  muscle  is  at  rest  when  he  is  looking 
at  any  object  20  feet  off,  or  beyond,  but  the  hyperope's 


Fig.  53. 

Showing  parallel  rays  focused  on  the  retina  of  a  hyperopic  eye 
by  means  of  a  convex  lens. 


eye  is  never  at  rest  if  he  attempt  to  see  distinctly; 
and,  moreover,  when  he  wishes  to  look  at  a  near  object, 
he  starts  with  a  deficit,  which  deficit  is  the  amount  of 
accommodation  he  required  for  distant  vision.  '  Thus, 
a  hyperope  of  four  dioptres,  with  five  dioptres  of 
accommodation,  can  focus  distant  objects  clearly, 
but  then  he  has  only  one  dioptre  left  for  near  vision; 
this  will  only  bring  his  near  point  to  i  metre  from  the 
eyes.  Again,  take  a  hyperope  of  two  dioptres,  with 
five  dioptres  of  accommodation:  he  has  only  three 
dioptres  available  for  accommodation  for  near  objects; 


88 


THE  REFRACTION   OF  THE  EYE 


this  brings  his  near  point  to  33  cms.,  but  he  is  using  the 
whole  of  his  accommodative  power  for  this,  and  it  is 
impossible  for  him  to  do  this  for  long  without  fatigue, 
and  so  we  get  all  the  symptoms  of  eyestrain. 

We  have  seen  that  in  hyperopia  a  =  p-(-r)  =  p+r 
(page   35),    therefore    p  =  a-r  ;    in    other    words,    the 


fO     /S     20     2S     50    3S     40     46     SO    66     GO     66      70      76    SO 


// 

m 

3 

\ 

H 

\ 

7 

\p 

fi 

\ 

5 

\, 

4- 

N 

J 

\ 

? 

k. 

•^/ 

Sj 

V 

a 

-/ 

--■ 

— 

•-- 

--- 

-- 

V 

^ 

-_. 

.__ 

--■ 

... 

.--. 

? 

'v 

,T 

v^ 

^ 

<t 

^ 

S 

6 

r 

" — 

^v^ 

P 

f 

r 

^ 

7 

\4 

8 

Fig.  54.* 

Showing  the  range  of  accommodation  of  an  uncorrected  hyperope 
of  4  D  at  different  ages. 

The  numerals  above  represent  years,  those  on  the  left,  dioptres. 
The  line  p  p  represents  the  curve  of  the  punctum  proximum,  and 
the  line  r  r  that  of  the  punctum  remotum. 

available  amount  of  accommodation  is  represented  by 
the  total  accommodation  less  the  amount  required 
to  correct  the  hyperopia,  so  that  although,  as  we  have 

*  As  Bonders'  diagrams  are  still  universally  used  I  have  re- 
tained them,  but  I  would  refer  the  reader  to  Chapter  IX. 
(Presbyopia). 


HYPEROPIA  89 

seen,  age  for  age  the  hyperopic  eye  has  the  same  total 
amount  of  accommodative  power  as  the  normal  eye, 
it  has  less  to  use  for  near  vision,  if  uncorrected.  In 
Fig.  54  the  amount  of  available  accommodative  power 
in  the  uncorrected  hyperope  of  4  d  is  represented 
by  the  number  of  dioptres  between  p  and  the  zero 
line;  thus,  at  the  age  of  30  we  find  only  two  and  a 
half,  representing  2»5  d  of  accommodative  power, 
because,  although  he  has  6*5  d  total  power  like  the 
emmetrope,  4  of  this  is  used  up  to  correct  his  defect. 
At  the  age  of  40  we  see  that  p  crosses  the  zero  line; 
in  other  words,  all  available  power  is  lost.  He  has 
4  D  left,  but  this  is  used  up  in  correcting  his  defect. 
Beyond  this  age  he  loses  still  more  accommodative 
power,  and  this  means  that  he  cannot  even  correct 
his  defect ;  in  other  words,  he  cannot  obtain  clear  images 
of  anything  far  or  near. 

This  condition  will  obtain,  of  course,  at  an  earlier 
age  if  the  hyperopia  be  higher.  Thus,  a  hyperope 
of  10  D  at  25  has  only  8  d  accommodative  power, 
and  consequently  sees  everything  indistinctly.  Such 
persons  often  approach  their  eyes  very  near  their  work 
in  order  to  obtain  larger  retinal  images,  and  an  erroneous 
diagnosis  of  myopia  is  liable  to  be  made. 

The  Varieties  of  Hyperopia. — The  hyperopia  which 
is  at  once  recognized,  the  patient  confessing  to  im- 
proved vision  with  a  convex  glass,  is  called  manifest, 
and  this  manifest  hyperopia  (Hm)  is  expressed  in 
amount  by  the  strongest  convex  glass  the  patient 
accepts.  For  instance,  a  patient  sees  |,  but  with  +1 
in  front  of  the  eye  f :  +i«5  makes  the  letters  hazy, 
then  +1  D  =  Hm. 

Again,  when  the  defect  is  hidden  by  the  patient 
using  his  accommodation  and  obtaining  perfect  distant 
vision,  hyperopia  is  present  if  he  accepts  a  convex 
glass  and  the  total  manifest  hyperopia  is  represented 
by  the  strongest  convex  glass  with  which  he  can  see 


90 


THE   REFRACTION   OF   THE   EYE 


Total 

Hyperopia 

(Ht) 


Latent  H. 
[HI) 


Manifest  H. 

[Hm) 


equally  well  as  with  the  naked  eye.  (It  should  be 
noted  that  neither  the  emmetrope  nor  the  myope  will, 
under  any  circumstances,  accept  even  the  weakest 
convex  glass  for  distance.) 

The  latent  hyperopia  is  the  additional  hyperopia, 
which  shows  itself  when  the  accommodation  has  been 
relaxed  by  a  cycloplegic.  If  the  patient  quoted  above, 
when  under  atropine,  see  |  only  when  +3  is  used,  in 
his  case  +2  represents  the  latent  hyperopia  (HI),  the 
total  hyperopia  (Ht)  being  the  sum  of  Hm  and  HI. 
Hm+HUHt. 

Thus: 

I  Only  revealed  under  a  cycloplegic. 

'Absolute  (Hma),  which  no  amount 
of  accommodation  can  correct, 
represented  by  the  weakest  con- 
vex glass. 
Facultative  (Hmf),  when  distant 
objects  can  be  clearly  seen  with 
or  without  convex  glasses,  repre- 
sented by  the  difference  between 
the  strongest  and  weakest  convex 
glass. 

The  want  of  harmony  between  the  accommodation 
and  the  convergence  is  a  constant  cause  of  eyestrain 
in  uncorrected  hyperopia.  We  have  seen  (page  48) 
that  in  normal  vision,  the  two  eyes  converging  for  a 
point  I  metre  off,  form  a  metre  angle,  and  use  i  d  of 
accommodation.  Eyes  converging  for  a  point  50  cms. 
off  have  to  converge  2  metre  angles  and  accommodate 
2  D,  and  so  on.  Now,  a  hyperope  of  2,  when  looking  at 
a  point  ^^  cms.  off,  is  using  W — i-^-y  3  d  added  to  the 
correction  of  his  hyperopia — viz.,  3+2  =  5  d;  but  he 
is  only  converging  3  metre  angles  instead  of  5,  con- 
sequently he  is  using  2  d  of  accommodation  in  excess 
of  convergence. 

Nature  has  endowed  many  hyperopes  with  the  power 
of  increasing  their  accommodation,  to  a  certain  extent, 
without  varying  their  convergence;  this  faculty  is  the 


HYPEROPIA  91 

result  of  "  nerve  education."  We  shall  see  when  deal- 
ing with  myopia  that  the  same  thing  occurs,  only  in 
this  case  it  is  the  convergence  that  is  used  in  excess  of 
the  accommodation.  There  is  a  minimum  amount  of 
effort  when  convergence  and  accommodation  work 
harmoniously  together,  as  it  were  supporting  each 
other ;  but  when  one  is  used  in  excess  of  the  other,  it  has 
to  work  unaided  and  alone,  and  strain  is  liable  to  ensue. 

Many  hyperopes  never  become  capable  of  using 
their  accommodation  in  excess  of  convergence,  and 
therefore  they  are  less  likely  to  suffer  from  strain 
(although  the  unconscious  effort  to  do  so  may  induce 
it) ;  but  a  worse  evil  befalls  them — they  lose  binocular 
vision,  and  squint.  A  hyperope,  under  these  circum- 
stances, finds  himself  in  the  following  dilemma:  if  he 
wishes  to  see  binocularly,  he  must  use  less  accon^mo- 
dative  power  than  he  requires  to  see  distinctly;  or, 
if  he  wishes  to  see  distinctly,  he  must  sacrifice  binocular 
vision,  which  ends  in  squint.  He  must  choose  between 
distinct  vision  and  binocular  vision.  Distinct  vision  is 
more  craved  for,  and  more  useful,  than  binocular  vision, 
especially  if  the  latter  be  not  quite  perfect,  owing  to 
one  eye  being  more  defective  than  the  other;  conse- 
quently, he  sacrifices  binocular  vision,  and  squints, 
and  the  eyestrain  ceases*  (see  Strabismus,  page  184). 

Conditions  causing  Hyperopia — i.  Axial  Hyperopia. 
— This  is  the  commonest  form,  and  is  the  condition  of 
most  eyes  at  birth.  It  is  due  to  the  shortening  of  the 
antero-posterior  diameter  of  the  eye,  and  may  be  due 
to  a  flattening  of  the  globe  or  to  a  general  diminution 
in  size.  Roughly,  every  3  d  of  hyperopia  represents  a 
diminution  of  i  mm.  of  the  axial  line. 

This  condition  is  due  to  an  arrest  of  growth  of  the 
eye,  and  is  often  associated  with  arrest  of  growth  of 
the  neighbouring  bony  parts;  thus,  the  face  of  a 
hyperope  often  shows  want  of  relief. 

*  Bonders  called  this  form  of  hyperopia  relative. 


92  THE   REFRACTION   OF  THE   EYE 

The  tendency  is  for  hyperopic  eyes,  at  birth,  to  grow 
towards  the  normal  and  even  to  become  myopic;  but 
after  50,  owing  to  the  increase  in  size  and  the  flatten- 
ing of  the  lens  with  age,  there  is  a  tendency  for  all  eyes 
to  become  hyperopic  as  life  advances;  this  is  called 
acquired  hyperopia  (see  page  I45)- 

2.  Curvature  Hyperopia  :  due  to  a  lack  of  convexity 
of  the  refractive  surfaces;  it  is  generally  associated 
with  astigmatism  (see  page  119). 

3.  Index  Hyperopia  :  due  to  diminution  in  the  index 
of  refraction  of  the  media. 

4.  Hyperopia  may  be  due  to  absence  of  the  lens 
{aphakia),  or  its  total  or  partial  dislocation. 

5.  Tumours  or  exudations  causing  advance  of  the 
retina  in  the  eye  will  cause  hyperopia. 

Symptoms. — It  is  the  facultative  hyperopia  which 
the  patient  can  correct,  and  thus  more  or  less  conceal  at 
will,  which  is  one  of  the  most  common  causes  of  eye- 
strain, and  the  reason  is  very  apparent. 

In  absolute  Hm,  vision  is  never  acute,  and  the  patient 
makes  no  attempt  to  strain  his  accommodation,  because 
he  finds  the  result  of  little  or  no  use. 

In  relative  Hm,  it  has  been  seen  that  only  monocular 
vision  can  be  acute,  and  that  eyestrain  generally  ceases 
when  the  squint  appears. 

Patients  with  facultative  Hm  are  most  frequently 
quite  unaware  that  they  are  suffering  from  any  defect 
of  the  eyes;  they  can  see  well  at  a  distance,  their  near 
vision  is  as  good  as  they  want,  they  have  no  idea  that 
the  headaches  that  come  on  after  near  work  are  caused 
by  eyestrain,  and  they  will  in  all  probability  be  treated 
for  all  manner  of  diseases  before  the  real  cause  is  dis- 
covered. It  is  true  that  when  this  eyestrain  lasts 
long,  signs  of  inflammation  will  often  show  themselves 
in  the  eye  and  its  appendages,  such  as  conjunctivitis 
and  blepharitis,  and  lead  the  patient  to  the  oculist; 
but  even  he  may  miss  the  true  cause  unless  he  make  it 


HYPEROPIA  93 

a  rule  to  examine,  under  atropine,  the  eyes  of  all  young 
people  suffering  from  chronic  inflammation  of  the  lids 
or  conjunctiva,  they  being  especially  the  subjects  of 
this  condition. 

The  facultative  hyperopia  of  the  young  becomes 
absolute  after  middle  life. 

Although  it  is  quite  possible  to  suffer  from  faculta- 
tive Hm,  and  pass  through  youth  without  any  symp- 
toms of  eyestrain,  sooner  or  later  they  will  appear. 
Good  health,  plenty  of  outdoor  exercise,  and  not  too 
much  application  to  books,  will  ward  off  eyestrain  for 
a  long  time;  but  in  these  days  of  examinations  the 
day  must  surely  come  when  the  young  student  must 
"  cram,"  when  he  must  read  four  or  five  hours  a  day 
by  artificial  light,  when  he  must  do  more  work  and 
take  less  play — in  other  words,  when  he  must  use  his 
muscles  of  accommodation  for  a  much  longer  time. 
As  a  result,  after  a  few  hours'  reading,  his  head  aches, 
his  eyes  pain,  and  the  type  appears  to  run  together. 
Many  such  cases  may  occur  from  simple  overwork  in 
emmetropes,  but  there  is  little  doubt  that  many  a 
young  man  has  broken  down  reading  for  his  "  Tripos  " 
simply  because  he  is  hyperopic  and  has  overstrained 
his  eyes.  Patients  often  suffer  from  eyestrain  for  the 
first  time  through  taking  up  German  or  Hebrew;  the 
fine  strokes  that  have  to  be  recognized  in  order  to  dis- 
tinguish the  different  letters  (especially  is  this  so  in 
Hebrew)  put  an  extra  strain  on  the  accommodation, 
and  if  the  eye  start  with  a  deficit,  as  it  does  in  hyper- 
opia, eyestrain  is  sure  to  ensue. 

If  symptoms  of  eyestrain  occur  among  the  upper 
classes  who  suffer  from  this  form  of  hyperopia,  how 
much  more  must  they  occur  in  those  who  spend  their 
lives  at  close  work,  in  badly  lighted  and  badly  venti- 
lated rooms,  with  little  or  no  outdoor  exercise  and  often 
insufficient  food.  Yet  the  large  body  ol  seamstresses 
and  compositors,  who  find  their  way  to  the  out-patient 


94  THE   REFRACTION    OF   THE   EYE 

room  of  an  ophthalmic  hospital,  are  only  a  small 
fraction  of  the  number  who  really  want  relief,  but 
do  not  recognize  it  because  they  see  well  without  glasses. 
Those  who  do  come  for  advice  generally  tell  the  same 
tale;  they  are  at  work,  say,  from  eight  in  the  morning 
till  eight  or  ten  at  night,  and  towards  evening  they 
complain  that  their  vision  becomes  less  acute,  and  their 
eyes  and  head  ache.  It  is  but  natural;  their  ciliary 
muscles  have  been  working  at  high  pressure  all  day,  and 
in  their  way  have  done  as  much  work  as  the  leg  muscles 
would  in  a  thirty-mile  walk.  Surely  in  an  emmetropic 
eye  we  should  expect  fatigue  under  such  circumstances ; 
how  much  more,  then,  in  a  hyperopic  eye  ! 

With  advancing  years  the  latent  hyperopia  becomes 
gradually  and  finally,  about  the  age  of  40,  entirely 
manifest,  and  with  the  diminution  of  accommodation 
range  the  hyperope  necessarily  becomes  prematurely 
"  presbyopic."  As  might  be  expected,  symptoms  of  eye- 
strain are  much  more  common  in  presbyopes  who  are 
hyperopic  than  in  those  who  are  emmetropic  or  myopic. 

One  of  the  results  of  eyestrain  in  young  hyperopes, 
or  in  those  who  have  to  make  great  efforts  to  see  small 
objects,  as  watchmakers,  is  spasm  of  the  ciliary  muscle, 
whereby  vision  is  accommodated  for  near  objects,  and 
the  patient  rendered  artificially  myopic  (see  page  109). 
This  spasm  is  usually  accompanied  by  a  contracted  pupil 
from  associated  spasm  of  the  sphincter  of  the  iris,  both 
conditions  being  caused  by  direct  or  indirect  irritation 
of  the  third  nerve  (see  page  201). 

Hyperopic  headache  is  often  accompanied  by  twitch- 
ings  of  the  eyelids. 

Besides  the  many  symptoms  of  eyestrain  already 
described,  the  special  indications  of  hyperopia  are — 

1.  Spasm  of  ciliary  muscle,  often  producing  apparent 
myopia. 

2.  Sudden  failure  of  the  ciliary  muscle  from  fatigue, 
causing  obscurations  of  vision. 


HYPEROPIA  95 

3.  Convergent  strabismus. 

4.  Apparent  divergent  strabismus. 

The  angle  gamma  is  larger — i.e.,  the  angle  formed  by 
the  visual  and  optic  axes  is  increased,  which  causes 
apparent  divergence  of  the  axis  of  the  cornea  (see 
page  187). 

There  are  also  certain  physical  signs  noticeable  in 
hyperopes. 

The  eye  is  flatter  than  normal  and  often  markedly 
smaller;  if  the  eyeball  be  turned  strongly  in  or  out,  the 
equatorial  region  presents  a  much  sharper  curve  than  in 
the  normal  or  myopic  eye,  showing  its  shortened  axis. 

The  cornea  is  often  smaller  than  usual.  The  face  is 
sometimes  flat-looking. 

The  ciliary  muscle  of  a  hyperope  is  always  larger  than 
normal ;  this  is  especially  marked  in  the  annular  muscle 
of  Miiller,  and  is  due  to  excessive  use. 

The  Diagnosis  of  Hyperopia  by  Examination — i.  Vision 
is  improved  by,  or  is  as  good  with,  convex  glasses. 

2.  Retinoscopy  gives  a  shadow  moving  "  with  "  with 
a  plane  mirror,  and  a  reverse  shadow  with  a  concave 
mirror,  and  the  more  defined  the  shadow  and  the  quicker 
its  movement,  the  lower  the  hyperopia. 

3.  The  indirect  ophthalmoscopic  examination  shows 
an  image  of  the  disc  larger  than  normal,  and  diminishing 
on  withdrawing  the  lens  from  the  eye. 

4.  In  the  direct  ophthalmoscope  examination,  if  the 
observer's  accommodation  be  relaxed,  a  convex  glass  is 
required  in  the  ophthalmoscope  to  obtain  a  clear  image 
of  the  fundus.  If  the  hyperopia  be  high,  the  mirror 
alone,  a  short  distance  from  the  eye,  shows  an  erect 
image  of  the  fundus  moving  with  the  observer. 

The  Influence  of  Age  on  Hyperopia. — See  Presbyopia, 
page  142. 

Treatment. — Up  to  6  years  of  age  atropine  should 
be  applied  to  the  eyes  for  at  least  three  days  twice  a 
day  before  the  examination. 


96  THE   REFRACTION   OF   THE   EYE 

During  this  period  of  life  the  eye  has  not  fully  de- 
veloped, most  children  have  a  certain  amount  of 
hyperopia,  which  should  only  be  corrected  if  high  in 
amount,  or  if  strabismus  be  present. 

As  the  examination  before  the  type  at  this  age  is  of 
no  practical  value,  the  surgeon  has  to  depend  on  the 
result  of  his  retinoscopy. 

The  glasses  ordered  should  be  weaker  than  the  atropine 
estimate.  Suppose  the  retinoscopy,  under  atropine, 
gives  +4  at  a  metre,  this  would  mean  +3  before  the 
type  if  the  patient  can  read,  and  the  glass  ordered 
should  not  be  stronger  than  +2.  It  will  be  found  that 
when  the  error  is  greater,  more  than  +1  must  be 
deducted  from  the  atropine  correction. 

The  correction  of  hyperopia,  when  adopted  with  those 
who  have  developed  convergent  strabismus,  has  often 
the  happiest  results,  for  the  squint  may  be  cured  with- 
out resorting  to  an  operation,  and  the  result  is  in  direct 
ratio  to  the  youth  of  the  patient.  Moreover,  these 
are  the  hyperopes  who  can  most  readily  be  made  practi- 
cally "  emmetropic,"  and  therefore  get  the  greatest 
gain  from  the  glasses ;  for  their  very  defect,  the  squint, 
shows  that  they  have  been  unable  to  dissociate  their 
accommodation  and  convergence.  It  is  this  habit  of 
using  their  accommodation  in  excess,  which  has  led  to 
hypertrophy  of  the  ciliary  muscle,  which  prevents 
many  hyperopes  from  taking  full  correction.  They 
cannot  overcome  the  habit,  and  naturally  the  older 
the  patient,  the  greater  the  probability  is  there  of 
this  being  possible.  In  such  cases  a  much  weaker 
convex  glass  must  be  given  at  first,  and  its  strength 
increased  later. 

When  the  amplitude  of  accommodation  is  great, 
the  patient  will  probably  prefer  a  weaker  glass  than 
we  wish  to  give,  and  vice  versa.  When  the  hyperopia 
is  small  in  amount,  and  equal  in  the  two  eyes,  and 
there  is  no  strabismus,  it  is  unwise  to  order  any  glass. 


HYPEROPIA  97 

From  ages  6  to  15  the  atropine  should  be  appUed 
to  the  eyes  twice  daily  for  two  days  before  exami- 
nation. 

By  this  time  the  eye  should  be  emmetropic,  and  as 
the  child  is  entering  the  active  period  of  school  career, 
the  hyperopia  should  be  corrected,  unless  very  small 
in  amount,  and  even  then  when  associated  with  astig- 
matism or  anisometropia,  or  both. 

At  this  period  we  must  recognize  the  "  personal 
equation."  Some  patients  will  stand  a  fuller  correction 
of  their  defect  than  others. 

After  ascertaining  the  amount  of  error  under  atropine, 
we  must,  whenever  possible,  arrange  for  a  final  examina- 
tion before  the  test  types,  when  the  effects  of  the  cyclo- 
plegic  have  passed  off.  We  should  prescribe  the  fullest 
correction  of  the  error  the  patient  will  accept,  consistent 
with  good  vision.  For  instance,  under  atropine  a  child 
of  12  reads  years  J  with  +3.  When  the  effects  of  the 
atropine  have  passed  off,  deducting  + 1  for  the  atropine, 
we  find  that  +  2  gives  J  hazily,  and  that  it  is  only  when 
we  reach  +i'25  that  §  is  clearly  read.  This  is  the 
correction  we  order,  with  a  cylinder  correcting  the 
astigmatism,  if  present  (see  page  138) . 

We  must  remember  that  such  a  patient  did  not  come 
to  us  for  improvement  in  vision;  he  probably  read  f 
quite  well,  and  if  we  insist  on  his  wearing  a  correction 
which  makes  his  distant  vision  worse,  he  will  seize  every 
opportunity  of  not  wearing  the  glasses. 

When  the  hyperopia  is  slight  (say  only  i»75  or  2  under 
atropine),  equal  in  the  two  eyes,  and  unassociated  with 
astigmatism  or  strabismus,  glasses  are  not  necessary. 

From  ages  15  /o  25  or  30  atropine  is  the  best  cyclo- 
plegic,  but  in  a  large  number  of  cases  we  have  to  be 
content  with  homatropine,  because  atropine  necessitates 
too  long  a  period  of  rest  from  work.  Glasses  should  not 
be  prescribed  until. the  patient  has  recovered  from  the 
effects  of  the  cycloplegic,  and  then  the  fullest  correction 

7 


98  THE  REFRACTION   OF  THE   EYE 

of  the  error  that  the  patient  will  accept,  consistent  with 
clear  vision,  should  be  ordered  to  be  worn  always.* 

In  high  hyperopia,  especially  in  cases  where  the  patient 
will  only  accept  a  very  partial  correction  of  the  error,  it 
is  advisable  to  give  the  full  atropine  correction,  as  special 
near- work  glasses,  for  use  by  artificial  light. 

From  ages  30  to  45  homatropine  should  be  used  as  the 
cycloplegic,  and  in  the  case  of  patients  with  high  hyper- 
opia, or  when  the  smallest  fear  of  glaucoma  is  present, 
one  drop  of  eserine  (2  grains  to  the  ounce)  should  be 
put  into  the  eyes  when  the  examination  is  finished. 

The  treatment  is  the  same  as  for  younger  patients. 

A  larger  proportion  of  the  correction  will  be  accepted, 
and  the  older  the  patient,  the  more  necessary  is  it  to 
give  the  full  cycloplegic  correction  for  near  work. 

Over  age  45  no  cycloplegic  is  necessary,  as  all  the 
hyperopia  has  by  this  time  become  manifest. 

The  strongest  convex  glass  the  patient  will  accept 
must  be  prescribed,  and  as  the  presbyopic  period  has 
arrived,  a  still  stronger  glass  will  be  required  for  reading, 
and  the  two  are  best  prescribed  in  one  glass  in  the  form 
of  invisible  bi-focals  (see  Presbyopia,  page  151)4 

*  When  the  patient  cannot  return  for  a  final  visit,  glasses 
correcting  the  whole  of  the  manifest,  and  about  one-third  of  the 
latent  hyperopia,  may  be  safely  prescribed.  (Manifest  hyperopia 
is  expressed  in  amount  by  the  strongest  convex  glass  the  patient 
accepts  when  not  under  atropine.  Latent  hyperopia  is  the  addi- 
tional hyperopia  which  is  revealed  under  atropine.) 

t  It  is  important  to  remember  that  hyperopia  tends  to  de- 
crease towards  emmetropia.  A  child  may  be  hyperopic  to  the 
extent  of  2  D,  and  as  the  growth  and  development  of  the  different 
parts  of  the  body  proceed,  the  flatness  of  the  eye  may  disappear, 
and  by  puberty  the  eye  may  have  become  emmetropic.  For 
this  reason  it  is  necessary  to  re-examine  the  eyes  of  children  at 
least  once  a  year,  in  order  to  make  sure  they  are  not  wearing  too 
strong  a  convex  glass,  which  would  induce  an  artificial  myopia, 
which  in  turn  might  lead  to  real  myopia.  This  tendency  for  the 
eye  to  grow  normal  is  often  interfered  with  by  the  presence  of 
eyestrain;  hence  one  of  the  benefits  derived  from  glasses  in 
young  children  is  the  increased  chance  of  the  eye  improving  and 
growing  to  the  normal  shape,  and  the  possibility  of  discontinuing 
the  use  of  spectacles. 


CHAPTER  VII 

MYOPIA 

Myopia  {fiveLv,  to  close,  o)\p,  the  eye,  from  the  habit  of 
myopes  to  partially  shut  the  eyes  in  order  to  lessen  the 
circles  of  diffusion),  or  short-sight,  is  a  condition  of  the 
eye  in  which  the  retina  is  situated  behind  the  principal 
focus  (Fig.  20,  m),  and  only  divergent  rays  from  a  near 
point  (Fig.  55),  or  parallel  rays  made  divergent  by  a  con- 
cave glass  (Fig.  56),  can  come  to  a  focus  on  the  retina. 


Fig. 


The  retina  of  a  myopic  eye  is  the  conjugate  focus  of 
an  object  situated  at  a  short  distance  in  front  of  the  eye, 
or,  in  other  words,  the  punctum  lemotum  of  a  myope  is 
always  at  a  definite  distance  (less  than  6  metres),  the 
distance  being  measured  by  the  amount  of  myopia. 
Thus,  a  myope  of  i  has  his  far  point  i  metre  from  the 
eye,  a  myope  of  2  has  his  far  point  J  metre,  or  50  cms., 
and  a  myope  of  5,  20  cms.,  from  the  eye. 

99 


100 


THE  REFRACTION  OF  THE  EYE 


A  myopic  eye  sees  distinctly  distant  objects  (when 
accommodation  is  relaxed)  with  that  concave  glass  whose 
focal  length  is  equal  to  the  distance  of  the  far  point  from 
the  eye,  and  the  converse  is  true:  the  measurement  of 
myopia  is  that  concave  glass  with  which  the  myopic 
eye  sees  distinctly  objects  at  a  distance,  and  its  focal 
length  is  equal  to  the  distance  of  the  myope's  far  point 
from  the  eye.  If  the  accommodation  be  relaxed, 
the  strongest  concave  glass  is  the  measure  of  the 
myopia. 

We  can  ascertain  the  punctum  remotum  of  a  myope 
directly  by  measuring  the  furthest  distance  at  which  he 


Fig.  56. 


can  sec  objects  distinctly ;  thus,  if  such  a  spot  be  2  metres 
from  the  eye,  this  is  R,  and  its  expression  in  dioptres  is 
r=  I  or  '5,  hence  the  myopia  =  -  •5. 

Myopia  may  be  produced  in  the  following  ways : 

1.  By  elongation  of  the  axis  of  the  eye — axial  myopia. 
This  may  be  due  to — 

(a)  General  elongation  of  the  eye — typical  myopia. 
(6)  Localized  protrusion  of  the  sclerotic,  particularly 
at  the  posterior  pole — staphyloma. 

2.  By  increase  of  the  refractive  power  of  the  eye — 
refractive  myopia. 

This  may  be  due  to — 

(a)  Increase  in  curvature  of  the  cornea,  as  in  myopic 
astigmatism. 


MYOPIA  10 1 

(b)  Increase  in  the  curvature  of  the  lens — 

(a)  In  spasm  of  the  accommodation. 
(/3)  In  luxation  of  the  lens. 

(c)  Increased  density  of  the  lens,  as  at  the  beginning 
of  senile  cataract. 

3.  A  combination  of  i  and  2,  as  in  conical  cornea, 
when  elongation  of  the  axis  and  increase  in  the  curvature 
of  the  cornea  coexist. 

Thus,  we  see  that  typical  myopia  is  due  to  an  elonga- 
tion of  the  antero-posterior  diameter  of  the  eye,  and 
every  dioptre  of  myopia  represents  a  lengthening  of  this 
axis  by  about  J  mm. 

The  punctum  remotum  and  punctum  proximum  of  a 
myope  are  ascertained  according  to  the  methods  already 
given. 

The  punctum  proximum  is  nearer  the  eye  than  in 
emmetropia,  and  the  higher  the  myopia  the  nearer 
it  is. 

As  r  has  a  positive  value  in  myopia,  the  amplitude 
of  accommodation  is  the  difference  between  p  and  r; 
thus  a=p-  r. 

The  Influence  of  Age  on  a  Myope.— See  Presbyopia, 
page  144. 

The  diagram  (Fig.  57)  represents  the  amplitude  of 
accommodation  of  a  myope  of  3  d  at  the  different  ages. 
The  Hue  ^  begins  at  the  figure  17,  showing  that  at  the  age 
of  10  the  near  point  is  6  cms.  from  the  eye;  therefore 
a  =  -§-  -  3  =  17  -  3  =  14  D.  At  30  years  of  age  ^  =  10  d, 
and  p  is  10  cms.,  while  R  is  still  33  cms.  on  the  positive 
side,  a  =  io-3  =  7  d. 

At  the  age  of  55  r  begins  to  curve  downwards,  and 
reaches  the  zero  line  at  80,  so  that  at  this  age  the  myope 
of  3  has  lost  all  his  myopia;  p  and  r  unite  (showing 
that  all  accommodation  is  lost)  at  the  age  of  75. 

The  Relation  between  Accommodation  and  Con- 
vergence.— A  myope  of  5  d  can  see  a  point  20  cms. 
from  the  eye  without  using  his  accommodation,but  he 


102 


THE  KEFRACTION  OF  THE  EYE 


must  converge  to  5  m.a.  in  order  to  see  binocularly.  As 
a  compensation  for  the  visual  defect,  most  myopes  have 
the  power  of  using  their  convergence  in  excess  of  their 
accommodation,  just  as  a  hyperope  has  often  the  power 
of  using  his  accommodation  in  excess  of  his  convergence ; 
but,  it  has  been  shown  (page  91),  they  both  have  to  pay 

Age. 
to    /S     20     25     30    35      40     4S     SO    SS    GO    €S     70     75  60 


/a 

n 

/€ 

\ 

/6 

\ 

/4- 

\ 

/3 

\ 

P 

/? 

// 

s 

/o 

\ 

9 

~\. 

<9 

N 

\ 

7 

N 

V 

6 

\ 

^ 

S 

\ 

V 

4 

V 

s 

J 

X 

^, 

2 

~r 

r- 

>s 

P 

*/ 

^ 

O 
/ 

--- 

-- 

--- 

-- 

'-- 

--- 

---- 

~ 

--- 

-- 

--- 

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2 

Fig.  57.* 

Showing  the  range  of  accommodation  of  an  uncorrected  myope 
of  3  D  at  different  ages.     (Donders.) 


a  penalty  for  this,  the  liabiUty  to  strain  always  being 
greater  when  either  effort  is  used  in  excess  of  the  other. 
The  "  fusion  supplement  "  must  be  greater  than  the 
emmetropia,  and  the  greater  the  "fusion  supplements^ 
the  greater  the  fatigue  to  the  internal  recti  ;  the  fatigue 
leads  to  "  insufficiency  "  of  the  muscles,  and  matters 
*  See  note,  p.  88, 


MYOPIA  103 

are  made  worse.  But  it  is  not  only  the  excess  of  con- 
vergence, but  the  excessive  convergence  that  tends  to 
produce  strain  and  fatigue  of  the  internal  recti.  The 
uncorrected  myope  sees  nothing  distinctly  beyond  his 
far  point,  and  when  he  wishes  to  see  clearly  he  must 
bring  everything  within  that  point;  for  instance,  an 
emmetrope  wishes  to  know  the  time  by  the  clock:  he 
can  see  the  clock  across  a  room,  but  the  myope  must 
go  up  to  the  clock  and  bring  it  within  his  far  point; 
and,  moreover,  the  incentive  to  use  this  remedy  is 
great  because  the  remedy  is  perfect.  A  high  hyperope 
has  the  same  difficulty  with  distant  objects,  but  he 
has  not  the  same  remedy.  Naturally,  the  greater  the 
myopia,  the  nearer  is  the  far  point,  and  the  greater  is 
the  convergence  strain. 

A  myope  requires  more  convergence  of  the  visual  lines 
because  vision  takes  place  closer  to  the  eyes,  and,  as 
Bonders  has  shown,  precisely  in  myopia  is  this  for  two 
reasons  more  difficult — first  on  account  of  impeded  move- 
ments, due  to  the  altered  shape  of  the  eyeball,  which 
becomes  elUpsoidal  in  form,  and  which  has  to  move  in  a 
cavity  of  similar  shape ;  and,  secondly,  on  account  of  the 
altered  direction  of  the  visual  lines,  the  angle  7  (angle 
formed  by  the  visual  and  optic  axes)  being  smaller  than 
in  emmetropia  or  hyperopia  (see  page  187).  If  a  myope 
cannot  dissociate  his  accommodation  and  convergence, 
he  has  the  same  difficulties  as  a  hyperope :  he  can  either 
see  distinctly,  but  sacrifice  binocular  vision  to  remove 
the  diplopia,  or  he  can  use  his  accommodation  when  he 
does  not  require  it,  and  see  indistinctly. 

From  the  observations  of  Bonders,  Nagel,  and 
Landolt,  we  find  that  the  relative  amplitude  of  accom- 
modation and  convergence  (see  page  52)  vary  consider- 
ably, not  only  according  to  the  refractive  error,  but  also 
in  different  individuals  with  the  same  error.  There  is 
a  tendency  for  the  accommodation  to  adapt  itself  to 
the  altered  state  of  refraction*  hence  most  myopes  can 


104  THE   REFRACTION  OF  THE  EYE 

converge  in  excess  of  their  accommodation;  and  when 
the  myopia  increases,  the  excess  of  convergence  over 
accommodation  also  increases. 

The  Causes  of  Myopia. — Although  myopia  is  heredi-. 
tary,  it  is,  with  few  exceptions,  not  congenital.  Almost 
all  eyes  are  hyperopic  at  birth. 

The  savage  is  rarely  myopic:  it  is  civilization  that  is 
responsible  for  it.  The  necessity  for  constantly  adapt- 
ing the  eye  for  near  objects  means  undue  convergence. 

Myopia  generally  first  shows  itself  from  the  age  of 
8  to  10,  when  school  work  begins  in  earnest — that 
is,  when  convergence  is  first  used  in  excess — and  there 
is  no  doubt  that  excessive  convergence  is  mostly  re- 
sponsible for  the  development  of  myopia.  The  over- 
used internal  recti  constantly  pulling  at  the  sclerotic 
(assisted  by  the  pressure  of  the  other  muscles)  tend  to 
lengthen  the  antero-posterior  diameter  of  the  eye,  be- 
coming then  the  most  potent  factor  in  the  causation  of 
myopia,  and  as  this  lengthening  of  the  antero-posterior 
axis  necessitates  still  greater  convergence,  a  vicious 
circle  is  produced,  and  the  myopia  tends  to  increase. 

The  hereditary  character  of  myopia  is  explained  by 
the  existence  in  such  eyes  of  an  "  anatomical  predisposi- 
tion "  to  myopia.  The  sclera  is  unusually  thin,  and  con- 
sequently less  able  to  resist  the  pull  of  the  internal  recti, 
and  the  relative  position  of  the  recti  and  the  position  of 
the  optic  nerve,  both  of  which  may  be  hereditary,  may 
be  factors  in  the  production  of  this  defect. 

Anything  which  causes  young  subjects  to  approach 
their  work  too  near  the  eyes  may  be  the  starting-point  of 
myopia.  Bad  illumination,  or  the  light  coming  from  the 
wrong  direction  (for  instance,  in  front),  or  defective  vision 
produced  by  corneal  nebulae,  or  lamellar  cataract,  etc., 
all  necessitate  over-convergence  in  order  to  obtain  clearer 
images,  and  myopia  may  be  produced. 

It  is  interesting  to  note  that  when  the  work  is  ap- 
proached very  near  the  eye,  but  convergence  is  not  used; 


MYOPIA  105 

as  in  the  case  of  watchmakers,  who  habitually  use  a 
strong  convex  glass  in  one  eye,  there  is  no  special 
tendency  to  myopia. 

Symptoms  and  Diagnosis  of  Myopia.  —  (i)  Distant 
objects  are  seen  indistinctly,  because  parallel  rays  focus 
in  front  of  the  retina  and  cross  and  form  diffusion  circles 
on  the  retina,  and  the  higher  the  myopia  the  larger  the 
diffusion  circles;  these  are  reduced,  commonly,  by  the 
myope  "  screwing  up  "  his  eyes,  and  in  later  life  by  the 
contraction  of  the  pupils. 

(2)  Near  objects  are  seen  distinctly,  and  the  near 
point  is  much  nearer  than  in  normal  eyes. 

(3)  Acuteness  of  vision  is  often  lowered,  and  in  high 
myopia  this  is  invariably  the  case,  because  the  stretching 
of  the  eye  leads  to  atrophic  changes  in  the  retina  and 
choroid. 

(4)  The  presence  of  convergence  insufficiency  and 
latent  divergence  (diagnosed  by  the  Maddox  test, 
page  44)  for  distant  and  near  objects,  often  becoming 
manifest  later,  and  ending  in  divergent  strabismus. 

(5)  An  apparent  convergent  strabismus  due  to  the 
angle  7  being  negative  (page  187). 

(6)  Many  of  the  symptoms  of  eyestrain,  but  not  so 
frequent  or  so  marked  as  in  hyperopia  (see  Heterophoria, 
page  171). 

(7)  Spasm  of  the  ciliary  muscle,  which  apparently  in- 
creases the  amount  of  myopia,  so  that  young  subjects 
will  choose  a  stronger  concave  glass  than  they  require. 

(8)  A  prominence  of  the  eyeball  is  sometimes  noted  in 
high  myopes,  but  is  not  always  present.  A  dilated  pupil 
and  dreamy  stare  are  sometimes  present. 

(9)  Muscse  volitantes  are  often  complained  of.  These 
are  probably  due  to  the  indistinct  vision  allowing  the 
vitreous  to  be  seen  against  a  hazy  background. 

(10)  In  high  myopia  vitreous  opacities  are  sometimes 
numerous  and  most  annoying. 

(11)  Myopes    often   stoop   very   much   and   become 


I06  THE   REFRACTION   OF  THE   EYE 

"  round-shouldered  "  from  their  habit  of  poring  over 
their  work,  and  this  stooping  at  near  work  tends  to 
produce  congestion  of  the  eyes  and  appendages. 

It  should  be  noted  that,  in  low  degrees  of  myopia,  often 
the  only  symptom  present  is  indistinct  distant  vision, 
and  this,  very  often,  is  not  recognized  by  the  patient  as 
a  defect.  Such  people  learn  to  recognize  indistinct  out- 
lines  by  the  aid  of  other  senses  in  a  way  that  emmetropes 
can  hardly  understand,  and  when,  in  later  life,  they  can 
put  off  the  wearing  of  glasses  for  near  work  for  many 
years,  or  till  extreme  old  age,  what  wonder  that  they 
and  their  relations  imagine  them  to  be  possessors  of 
remarkably  good  sight  ! 

The  Diagnosis  of  Myopia  by  Examination. 

Objective  Examination — The  Ophthalmoscope  ;  (i)  The 
Indirect  Method, — By  this  method  the  disc  appears 
smaller  than  in  emmetropia.  On  withdrawing  the  focus- 
glass  from  the  patient's  eye  the  disc  becomes  larger. 
Without  the  focus-glass,  in  high  myopia  the  fundus 
is  seen  very  large  and  inverted,  if  the  observer  be  not 
nearer  the  aerial  image  than  his  own  near  point,  and  if 
the  observer's  head  be  moved,  the  image  of  the  disc 
appears  to  move  in  an  opposite  direction. 

(2)  The  Direct  Method. — By  this  method — viz.,  with 
the  ophthalmoscope  close  to  the  eye — the  fundus  is 
indistinct,  and  the  concave  glasses  have  to  be  rotated  in 
front  of  the  opening;  the  weakest  concave  glass  that 
gives  a  distinct  image  is  the  measure  of  the  myopia,  if 
the  observer's  accommodation  is  relaxed. 

Retinoscopy. — With  a  plane  mirror  the  shadow  moves 
"  against,"  and  with  a  concave  mirror  "  with,"  if  the 
observer  be  beyond  the  patient's  far  point.  With  the 
observer  seated  i  metre  from  the  patient,  the  measure  of 
myopia  is  that  concave  lens  which  gives  the  point  of 
reversal,  with  -id  added  (see  page  79). 


THE   FUNDUS   OF    THE    RIGHT   EYE   OF   A   MYOPE. 

The  amount  of  myopia  is  9D,  and  correction  gives  normal  vision. 

The  retinal  vessels  are  very  straight,  and  they  are  seen  to  curl  over 
the  tilted  nasal  margin  of  the  disc. 

The   "  myopic   cre,scent,"    instead   of   being   limited    to  the  outer  or 
temporal  side  of  the  disc,  is  seen  to  surround  it. 

[The  fellow  eye  is  much  the  same,  only  the  changes  at  the  macula 
are  more  marked,  and  vision  is  only  j'j]. 


MYOPIA  107 

Subjective  Examination.  —  Having  ascertained  the 
amount  of  error  by  the  above  methods,  we  seat  the 
patient  before  the  test  types,  and  proceed  to  correct  with 
concave  lenses  in  the  trial  frame. 

As  the  accommodation  is  never  so  relaxed  as  when  the 
eye  is  under  the  influence  of  a  cycloplegic,  the  patient 
may  require  a  slightly  stronger  lens  than  the  objective 
examination  indicated.  Thus,  if  retinoscopy  at  a  metre 
from  the  patient  gives  us  the  point  of  reversal  with  -  5, 
we  call  the  amount  of  defect  -  6,  and  before  the  types 
the  patient  may  prefer  -  6'5. 

Note  the  weakest  lens  that  gives  the  most  distinct 
vision  with  each  eye  separately,  and  then  try  the  glasses 
binocularly  when  binocular  vision  exists,  as  sometimes 
the  patient  will  accept  and  prefer  a  slightly  weaker  glass. 

As  the  circles  of  diffusion  are  removed  by  correction, 
the  myope  often  finds  that  concave  glasses  reduce  the 
size  of  objects.  In  Fig.  21,  page  26,  m  represents  the 
position  of  the  myopic  retina,  and  the  image  of  an 
object  on  it  is  seen  to  cover  a  larger  area  than  the  same 
object  does  either  in  emmetropia  or  hyperopia,  H. 

Changes  in  the  Fundus  in  Myopia. — In  most  myopes  a  white 
crescent  is  observable  on  the  temporal  side  of  the  disc;  this  is 
the  myopic  crescent.  It  may  not  be  limited  to  this  part,  and  may 
even  surround  the  disc  (see  frontispiece).  In  high  myopia  it 
often  extends  on  the  outer  side  towards  the  macula.  Its 
margins  are  often  pigmented.  This  crescent,  which  is  a  localized 
atrophy  of  the  choroid,  is  brought  about  by  the  stretching  of 
the  tunics  in  the  formation  of  the  posterior  bulging  or  staphy- 
loma posticum  of  the  eyeball.  The  "  dragged  disc  "  is  due  to  the 
resistance  of  the  optic  nerve  (often  shorter  than  normal)  on  the 
one  side,  and  the  posterior  staphyloma  on  the  other.  Whether 
the  atrophy  is  secondary  to  choroiditis  or  merely  due  to  insuffi- 
cient nutrition,  it  is  difficult  to  say.  Von  Graefe  asserted  that 
the  staphyloma  posticum  is  due  to  a  sclerotico-choroiditis.  If 
this  be  so,  then  this  particular  spot  bulges,  because  it  is  unsup- 
ported by  the  recti  which  compress  the  sides  of  the  globe.  This 
also  explains  why  undue  convergence,  by  increasing  the  intra- 
ocular pressure  by  pressing  the  recti  on  the  globe,  is  such  an 
important  factor  in  causing  and  increasing  myopia.  The  dragged 
or  tilted  disc  is  very  characteristic,  and  becomes  deeply  cupped 
in  some  cases.     This  cupping  is  quite  distinct  from  glaucomatous 


I08  THE  REFRACTION  OF  THE  EYE 

cupping,  in  that  it  is  most  marked  on  the  nasal  side,  the  vessels 
rising  up  and  dipping  down  over  the  tilted  edge  in  a  very  charac- 
teristic fashion,  and  it  does  not  occupy  the  whole  area  of  the  disc 
(see  frontispiece). 

If  the  myopia  progress,  the  changes  may  become  general, 
and  after  a  time  white  patches  of  choroidal  atrophy,  with  masses 
of  black  pigment  forming  their  boundary,  are  scattered  all  over 
the  fundus.  These  changes  extend  to  the  vitreous,  causing 
liquefaction  of  that  body  and  the  subsequent  shrinking,  and  the 
consequent  loss  of  support  to  the  retina  may  end  in  detachment 
of  that  membrane.  Unfortunately,  very  often  some  of  the  most 
serious  changes  occur  at  the  macula,  as  this  is  the  region  of  the 
bulging,  and  haemorrhages  and  consequent  atrophy  lead  to  a 
result  as  disastrous  as  the  detachment. 

Some  cases  of  high  myopia  have  been  termed  "  malignant," 
and  it  is  very  probable  that  many  of  them  ought  not  to  be  classed 
under  myopia  at  all,  but  that  the  myopia  is  only  a  symptom  of 
a  disease  attacking  the  eye. 

The  milder  cases  of  progressive  or  malignant  myopia  (wrongly 
so  called)  are  often  the  result  of  wrong  treatment,  as  we  shall  see. 

Treatment— 

Donders  said:  "The  effect  of  wearing  glasses  is,  in 
fact,  that  the  relative  range  of  accommodation  is 
displaced,  becoming  gradually  the  same  as  the  position 
proper  to  emmetropic  eyes,  and  therefore  the  binocular 
furthest  point  approaches  the  eye  while  the  absolute 
furthest  point  r  by  no  means  does  so.  The  myopia 
thus  neutralized  is  less  progressive,  because  both  too 
strong  convergence  and  a  stooping  position  are  avoided." 

Since  his  day  some  ophthalmologists  have  advocated 
the  practice  of  allowing  those  suffering  from  low  myopia 
to  do  near  work  without  glasses,  and  when  the  myopia 
was  high,  have  given  weaker  glasses  for  near  work. 
The  consequence  has  been  that  as  the  convergence  was 
still  being  used  in  excess,  the  myopia  tended  to  progress. 
If  Bonders'  teaching  had  been  followed,  this  error 
would  never  have  been  made,  an  error  which  has  kept 
progressive  myopia  and  malignant  myopia  dreaded  for 
so  many  years. 

Following  on  Bonders'  teaching,  and  making  use  of 
our  increased  knowledge,  which  indicates  the  im- 
portance of  correcting  low  errors  of  astigmatism,  we 


MYOPIA  109 

have  made  great  advances  in  recent  years.  The  full 
correction  of  the  error  (with  the  smallest  minus  cylinder) , 
except  in  cases  of  very  high  myopia,  leads  not  only  to 
the  arrest  of  the  progress  of  the  myopia,  but  in  some 
cases  to  its  distinct  diminution. 

In  a  paper  read  in  1904  at  the  British  Medical  Association 
meeting  at  Oxford,  I  cited  532  myopes  who  had  been  treated 
by  full  correction. 

The  myopia  ranged  from  '75  to  20,  and  the  average  period  of 
observation  was  four  and  a  half  years.  The  following  table 
shows  the  result : 

'469  remained  stationary,  and  of  these,  in  162,  the  visual 
acuity  improved. 
532<|     63  progressed.  f  Increase  limited  to  i  d,  47. 

Average  age,  15.  <^         ,,  ,,2  D,  13. 

1^  M.  from  -I  to  -II.    [        ,,  ,,         4  D,     3. 

If  we  exclude  the  27  whose  increase  was  limited  to  i  d,  we 
have  16  left — i.e.,  only  3  per  cent,  progressing. 

In  early  life  the  treatment  of  myopia  is  mostly 
preventive.  It  is  rare  to  come  across  a  child  under 
the  age  of  6  with  actual  myopia.  Apparent  myopia 
may  show  itself — (a)  from  spasm  of  the  ciliary  muscle, 
distant  vision  being  subnormal  and  improved  by  con- 
cave glasses,  and  near  work  being  approached  very 
close  to  the  eyes;  (b)  in  a  young  patient  with  high 
hyperopia,  in  which  case  distant  vision  is  poor,  and  near 
work  is  held  very  near  to  the  eyes  in  order  to  acquire 
large  retinal  images;  (c)  in  children  who  have  acquired 
the  habit  of  holding  their  work  near  the  eyes,  either 
through  faulty  illumination  or  on  account  of  reduced 
visual  acuity,  produced  by  some  disease  of  the  eyes,  such 
as  corneal  nebulae. 

In  all  these  cases  the  true  error  is  revealed  by  a  cyclo- 
plegic,  consequently  no  attempt  should  ever  be  made  to 
treat  a  young  myope  without  previously  paralyzing  the 
accommodation,  and  atropine  should,  if  possible,  always 
be  used. 

The  preventive  treatment  is  more  especially  indicated 
in  all  children  whose  parents  are  myopic,  for  they  have 


I3t0  tHfi  REFRACTION   OF  THE  EYE 

probably  inherited  an  "  anatomical  predisposition  *'  to 
myopia. 

Bearing  in  mind  that  excessive  convergence  is  the 
most  potent  cause  of  myopia,  the  most  rigid  attention 
should  be  paid  to  ophthalmic  hygiene.  The  schoolroom 
should  be  lofty  and  large,  and  have  high  windows  on  one 
wall.  The  seats  and  desks  should  be  arranged  in  rows 
so  that  the  students  sit  with  the  windows  on  their  left. 
When  practicable,  each  scholar  should  have  an  adjust- 
able seat  and  desk,  but  when  this  cannot  be  arranged^ 
as  most  children  of  the  same  age  are  of  the  same  height 
while  sitting,  and  in  the  same  class,  the  height  of  the 
desk  from  the  seat  should  increase  gradually  with  the 
classes,  the  highest  class  having  the  highest  desks.* 

The  school  work  that  needs  close  application  of  the 
eyes  should  be  continued  only  for  a  short  period  at  a 
time,  the  period  alternating  with  other  work  which  does 
not  require  the  use  of  the  eyes,  such  as  mental  arithmetic^ 
demonstrations,  or  play. 

Schoolmasters  should  teach  more — that  is,  they  should 
explain  and  impart  knowledge  by  demonstrations  and 
simple  lectures,  and  reduce  as  much  as  possible  the  time 
spent  in  "  home  preparation,"  which  is  usually  work 
done  by  bad  light,  and  when  the  student  is  physically 
and  mentally  tired. 

Even  in  the  nursery  the  greatest  care  should  be  taker! 
with  the  children's  sight.  They  should  have  large  toysy 
and  among  these  there  should  always  be  a  large  box 
of  plain  wooden  bricks;  picture-books  should  be  dis- 
couraged, and  close  work  that  entails  undue  converg-^ 
ence,  such  as  sewing,  threading  beads,  etc.,  should  be 
forbidden.  The  nursery  governess  can  teach  them  theif 
letters  and  small  words,  and  even  simple  arithmetic,  by 
means  of  the  wooden  bricks. 

*  The  desk  should  have  a  slight  slope,  and  its  height  should  be 
so  regulated  that  the  scholar  can  sit  with  head  upright  and  the 
eyes  about  33  cms.  from  the  work. 


MYOMA  Itl 

No  child  with  a  tendency  to  myopia  or  with  a  myopic 
family  history  should  be  allowed  to  learn  to  write  or  to 
draw  until  at  least  7  years  old. 

The  child's  bed  should  not  be  allowed  to  face  the 
window;  preferably  it  should  be  back  to  the  Ught. 

Having  ascertained  the  concave  glass  that  corrects  the 
myopia  of  each  eye  under  atropine,  we  may,  in  quite 
young  patients,  order  such  glasses  for  constant  use;  in 
those  over  puberty  it  is  wise  to  delay  prescribing  until 
the  effects  of  the  atropine  have  passed  off — not  only 
because  an  increase  of  '5  may  very  distinctly  improve 
vision,  but  because  it  is  important  to  try  the  glasses 
binocularly  when  the  eye  is  in  the  normal  state.  It  is 
too  often  forgotten  that  the  eyes  are  not  single  optical 
instruments,  and  we  often  find  that  a  weaker  pair  of 
concave  glasses  give  as  good  vision  as  a  stronger  glass 
used  monocularly. 

The  only  certain  method  of  arresting  the  progress  of 
myopia  is  to  establish  a  normal  state,  in  which  the 
ciliary  muscle  is  strengthened  by  being  forced  to  work, 
the  excess  of  convergence  over  accommodation  stopped, 
and  excessive  convergence  made  impossible,  and  this  can 
only  be  achieved  by  insisting  upon  the  constant  use  of 
the  glasses,  and  refusing  to  give  weaker  ones  for  near  work. 
The  patient  can  see  his  near  work  so  much  better  without 
glasses  that  we  may  have  some  trouble  at  first  in  enforcing 
this  treatment. 

Of  course,  the  precaution  must  be  taken  of  re- 
moving the  glasses  when  rough  games  are  being 
played. 

In  adults  the  treatment  of  myopia  should  be  carried 
out  in  the  same  manner,  substituting  homatropine  for 
atropine  in  those  who  cannot  afford  the  time  from  work 
that  the  latter  entails. 

If  the  myopia  be  somewhat  high,  say  6  d  or  over,  and 
has  never  been  fully  corrected,  we  may  have  to  give 
glasses  for  near  work  1*5  or  2  d  weaker,  but  the  patient 


112  THE  REFRACTION   OF  THE  EYE 

should  be  strongly  advised  only  to  wear  these  on  special 
occasions  when  fine  work  is  being  done,  or  by  artificial 
light. 

The  older  the  patient  and  the  higher  the  myopia  the 
more  difficult  will  it  be  for  him  to  use  the  distance 
glasses  for  near  work,  because  his  accommodation  has 
been  so  long  idle  that  the  ciliary  muscle  is  considerably 
atrophied. 

If  the  patient  be  a  student  or  engaged  in  literary  or 
other  work  which  entails  close  application  for  many 
hours  a  day,  and  if  he  be  free  to  regulate  his  work,  he 
should  be  advised  to  work  for  shorter  periods  and  take 
longer  intervals  of  rest,  and  be  especially  careful  to  have 
his  work  always  in  a  good  light. 

In  patients  of  30  years  of  age  and  up  to  40,  homatro- 
pine  should  be  used  when  practicable.  Over  40  no 
cycloplegic  is  required.  If  the  patient  has  never  had 
the  full  correction,  he  will  at  this  age  be  unable  to  read 
with  his  distance  glasses,  and  weaker  ones  must  be  given, 
preferably  in  the  form  of  bi-focals.  All  the  more  will  this 
be  the  case  when  he  arrives  at  40  or  45,  the  emmetrope's 
presbyopic  period ;  no  rigid  rule  should  be  observed,  but 
each  case  should  be  treated  according  to  its  requirements. 

After  carefully  testing  the  patient,  we  should  find  his 
working  near  point,  and  keep  more  accommodation  in 
reserve  than  would  be  required  in  the  emmetrope  because 
the  ciliary  muscle  is  weaker  (see  Presbyopia,  page  150). 

Some  adults  with  a  small  amount  of  myopia  obsti- 
nately refuse  to  wear  the  constant  correction:  ladies 
will  wear  lorgnettes  at  the  theatre,  etc. ;  men  will  wear 
a  monocle.  If  no  astigmatism  be  present,  this  may 
be  allowed,  so  long  as  no  increase  in  the  myopia  takes 
place,  but  only  on  the  condition  that  the  patient  is 
re-examined  at  frequent  intervals. 

ffigh  Myopia. — ^The  treatment  of  high  myopia  is  some- 
what different.  When  the  young  adult  has  never  worn 
the  full  correction,  it  will  be  useless  to  prescribe  it,  even 


MYOPIA  113 

for  distance,  at  first.  We  should  reduce  the  glass  as 
little  as  possible,  and  test  binocularly.  For  instance,  the 
myopia  may  be  20  in  both  eyes,  but  -  18  before  each 
eye  is  the  strongest  glass  the  patient  will  tolerate.  These 
we  order  for  constant  use,  and  we  may  find  that  later 
the  full  correction  will  be  accepted.  In  older  patients, 
not  only  have  we  to  be  satisfied  with  a  reduction  in  the 
distance  glasses,  but  we  must  often  take  off  as  much  as 
4  D  for  near  work. 

Adults  who  have,  say,  10  d  of  myopia,  will  very  often 
refuse  to  wear  this  correction  because  of  the  discomfort 
entailed;  in  such  cases  it  will  be  found  that  the  best 
method  is  to  give,  say,  8  d  for  constant  use,  and  to 
give  them  2  d  either  as  lorgnettes  or  as  a  "  spy  "  glass 
to  put  up  in  front  of  the  8  d  when  they  want  particularly 
to  see  at  a  distance. 

The  advantage  of  this  treatment  is  that  the  "  constant " 
glass,  being  weaker,  may  do  quite  well  for  reading. 

When  recent  fundus  changes  are  present  in  young 
patients,  the  eyes  should  be  kept  under  atropine  for  a 
long  period,  the  correcting  glasses  should  be  well  tinted, 
or  made  with  Crookes's  "  B  "  glass  (see  below),  and  a 
country,  open-air  life  should  be  strongly  recommended, 
with  complete  cessation  of  all  close  work  while  the 
changes  are  active;  older  patients  should  be  warned 
against  stooping  or  straining,  and  should  be  strongly 
advised  to  do  little,  if  any,  near  work.  In  all  these  cases 
so  much  depends  on  the  general  health  that  it  is  wise  for 
the  surgeon  to  place  them  under  the  care  of  a  physician, 
who,  among  other  things,  can  advise  as  to  aperients,  the 
means  of  reducing  high  blood-pressure  when  present, 
etc.,  and  by  this  care  we  may  avert  retinal  haemorrhage 
which  is  so  liable  to  occur. 

When  any  fear  exists  as  to  the  possibility  of  detach- 
ment of  the  retina  ensuing,  the  patient  should  be  especi- 
ally warned  against  riding  on  horseback,  jumping,  or 
doing  any  act  which  may  jar  the  body. 


114  THE  REFRACTION  OF  THE  EYE 

The  Treatment  of  High  Myopia  by  Discission  and 
Removal  of  the  Lens. — When  the  patient  is  not  older 
than  25,  the  myopia  very  high,  vision  very  poor,  and  not 
improved  by  glasses  beyond  yt>  and  when  the  fundus 
changes  are  not  very  marked,  and  especially  when  the 
myopia  is  progressive,  the  lens  may  be  removed  by 
discission.  The  improvement  is  sometimes  very  great, 
the  patient  being  able  to  see  better  without  a  glass  than 
he  did  previously  with  a  strong  concave  lens.  (For 
details  of  this  treatment  the  reader  should  consult  a 
textbook  on  ophthalmology.) 

Crookes's  Glass  (see  page  211). — It  is  advisable  to  have 
the  correction  of  myopes  made  with  Crookes's  "  A  "  glass. 
In  high  myopia  this  special  glass  is  imperative,  and 
where  the  myopia  is  progressive  Crookes's  "  B  "  should 
be  used. 


CHAPTER  VIII 

ASTIGMATISM 

In  discussing  errors  of  refraction,  it  has  been  shown 
that  both  hyperopia  and  myopia  are  mainly  due  to  an 
alteration  in  the  shape  of  the  eye  as  a  whole,  the  antero- 
posterior axis  being  too  short  or  too  long — axial 
ametropia;  but  the  fact  was  also  mentioned  that  altera- 
tions in  the  curvature  of  the  cornea  or  lens  would 
produce  these  errors,  that  if  the  curvature  were  too  fiat. 


Fig. 


parallel  rays  would  focus  beyond  the  retina,  and  if  too 
great,  in  front  of  the  retina.  It  is  these  errors  of 
curvature  that  will  now  be  considered. 

In  the  normal  standard  eye,  if  an  opaque  disc  with  a 
sUt  aperture  (Fig.  58)  be  placed  in  front  of  it,  at  whatever 
angle  this  slit  is  rotated,  distant  objects  will  be  seen 
through  it  distinctly — that  is,  parallel  rays  will  focus  on 
the  retina.     In  simple  hyperopia  and  myopia  the  same 

"5 


Ii6 


THE   REFRACTION   OF   THE    EYE 


result  is  obtained  after  correcting  the  ametropia  with  a 
spherical  lens,  because  the  surfaces  of  the  dioptric  ap- 
paratus are  perfect  spheres,  and  consequently  all  the 
meridians  have  the  same  curvature.  But  suppose  we 
examine  an  eye  in  which  the  vertical  meridian  is  normal 
— i.e.,  parallel  rays,  passing  through  this  meridian,  focus 
on  the  retina — but  in  which  the  horizontal  meridian  has  a 
flatter  curvature,  then  parallel  rays  passing  through  this 
flatter  curvature  will  focus  beyond  the  retina,  and  as  they 
impinge  on  the  retina  will  form  diffusion  circles.  In 
such  a  case  all  the  meridians,  between  the  horizontal  and 


o  -  O   I 


Fig.  59. 


vertical,  will  have  a  different  focus  point,  such  points 
gradually  approaching  the  retina  as  the  meridian  becomes 
more  vertical. 

Such  an  eye  is  astigmatic,  and  astigmatism  may  be 
defined  as  an  ametropia  of  curvature,  a  condition  in 
which  rays  of  light,  passing  through  the  dioptric  appar- 
atus, do  not  all  focus  at  one  point. 

In  regular  astigmatism,  of  which  the  above  is  an 
example,  and  which  is  now  under  discussion,  the 
meridians  of  greatest  and  least  curvature  are  always  at 
right  angles  to  each  other,  and  are  called  the  principal 

*  For  the  sake  of  simplicity,  the  two  principal  meridians  only 
are  shown. 


ASTIGMATISM  II7 

meridians;  the  meridians  in  between  these  have  a 
greater  or  less  curvature,  according  as  they  are  nearer 
to  the  former  or  to  the  latter.  The  meridian  exactly 
between  the  two  (corresponding  to  an  angle  of  45°  if  the 
meridians  of  greatest  and  least  curvature  are  vertical 
and  horizontal)  has  its  focus  point  exactly  between  that 
of  the  greatest  and  that  of  the  least  curvature. 

The  bowl  of  a  spoon  is  an  exaggerated  example  of  an 
astigmatic  surface,  the  curve  from  side  to  side  being 
much  greater  than  that  from  the  handle  to  the  tip  of  the 
spoon. 

Fig.  59  shows  in  a  highly  diagrammatic  manner  the 
shape  of  the  images  formed  by  a  regular  astigmatic  sur- 
face. Let  the  vertical  meridian  have  a  curvature  a  b, 
so  that  parallel  rays  passing  through  it  focus  at  /.  Let 
the  horizontal  meridian  c  d  have  a  flatter  curvature,  so 
that  rays  passing  through  it  focus  beyond  /  at  /'. 

If  a  beam  of  light  pass  through  such  a  surface,  and  the 
resulting  cone  of  light  be  intercepted  at  the  different 
positions  i  to  5,  the  image  will  be  altered  in  shape 
according  to  its  position. 

At  /  the  vertical  rays  have  come  to  a  focus,  and 
therefore  form  a  point  of  light ;  but  the  horizontal  rays 
have  not  come  to  a  focus,  and  will  be  spread  out,  as  at 
(2),  into  a  horizontal  line,  called  the  anterior  linear  focus. 
The  reverse  obtains  at  f\  as  shown  at  4,  because  the 
horizontal  rays  have  come  to  a  focus,  and  the  vertical 
have  crossed  and  form  diffusion  circles  and  spread  out 
the  points  of  light  into  a  vertical  Ine,  called  the  "  pos- 
terior linear  focus."  At  i  none  of  the  rays  have 
focused,  but  the  vertical  are  nearer  the  focus  than  the 
horizontal,  so  the  figure  here  will  be  an  oblate  ellipse. 
At  3  the  vertical  rays,  having  crossed,  are  diverg  ng  as 
much  as  the  horizontal  are  converging,  and  here  the 
figure  is  a  circle.  At  5  the  vertical  rays  are  more  out  of 
focus  than  the  horizontal,  so  that  the  figure  is  a  prolate 
ellipse. 


Il8  THE   REFRACTION   OF  THE  EYE 

When  the  retina  is  situated  at  any  of  these  positions 
(i  to  5),  the  image  on  the  retina  will  be  something  like  the 
diagrammatic  sketch.  The  interval  between  /  and  /' — 
i.e.,  between  the  focal  points  of  the  principal  meridians — 
is  called  the  "  focal  interval  of  Sturm,"  and  represents 
the  amount  of  astigmatism. 

The  vision  of  an  astigmatic  person,  when  the  astigma- 
tism is  sufficiently  high  to  cause  a  defect  of  vision,  is 
different  from  that  of  the  defective  vision  of  the  hyperope 
or  myope.  Objects  may  not  appear  blurred  generally, 
but  only  in  parts ;  lines  are  lengthened  or  broadened, 
and  circles  appear  elliptical.  He  may  be  able  to  read 
some  letter  in  §,  but  even  in  line  ys  he  may  not  read  all 
correctly;  he  supplies  the  visual  deficiency  by  guessing. 


Fig.  6o. 

In  every  eye  affected  with  regular  astigmatism  there 
is  one  direction  in  which  straight  lines  appear  most 
dist  nct,|and  another  at  right  angles  to  it  in  which  the 
line  ismost  indistinct;  hence,  if  two  lines  at  right  angles 
to  each  other  are  held  before  an  astigmatic  eye,  they 
cannot  both  be  distinct:  if  one  is  in  focus,  the  other  is 
b  urred. 

In  Fig.  59,  where  the  vertical  merid  an  is  more  sharply 
curved  than  the  horizontal,  at  the  anterior  Unear  focus  (/) 
a  horizontal  line  will  appear  in  focus,  but  a  vertical  line 
blurred;  and  at  the  post-linear  focus  (/')  a  vertical  line 
will  appear  in  focus  and  a  horizontal  line  blurred;  they 
cannot  both  be  in  focus  at  the  same  time. 


ASTIGMATISM  II 9 

Thus,  when  two  lines  at  right  angles  to  each  other  (a 
and  B,  Fig.  60,  a)  are  looked  at  by  an  eye  affected  with 
simple  astigmatism,  if  the  vertical  meridian  be  defective, 
A  will  appear  defined  and  B  blurred,  because  A  is  spread 
out  vertically  and  this  does  not  affect  the  definition, 
while  the  vertical  "  spreading  out  "  of  B  makes  the  line 
appear  blurred  (Fig.  60,  h).  If  the  horizontal  meridian 
be  defective,  the  reverse  happens  (Fig.  60,  c). 

Varieties  of  Regular  Astigmatism  (Fig.  6i) : 

v„_:^* CK  »• r  —  Refraction  of  the  Position  of  the 

Variety  of  Astigmatism.        principal  Meridians.  Principal  Focus. 

1.  Hyperopic  Astigmatism — 

(a)  Simple.         /Emmetropic.  On  the  retina 

^  '  ^  \  Hyperopic.  Behind  the  retina. 

{b)  Compound.  Both  hyperopic.  Both  behind  the  re- 
tina, one  being  nearer 
than  the  other. 

2.  Myopic  Astigmatism — 

(n\  QimT-ii^  /Emmetropic.  On  the  retina. 

W  simple.         ^Myopic.  In  front  of  the  retina. 

(6)    Compound.     Both  myopic.  Both  in  front  of  the 

retina,  one  nearer 
than  the  other. 

3.  Mixed  AsTiGMA-     f  Hyperopic.  Behind  the  retina. 

TiSM.  \ Myopic.  In  front  of  the  retina. 

Generally,  the  vertical  meridian,  or  one  near  it,  is  most 
convex,  and  this  is  called  "  direct  astigmatism."  Thus, 
in  direct  astigmatism  the  horizontal  meridian  (or  one 
near  it)  is  hyperopic  in  simple  and  mixed  astigmatism 
and  most  hyperopic  in  compound  hyperopic  astigmatism, 
and  the  vertical  meridian  (or  the  one  near  it)  is  myopic 
in  simple  and  mixed  astigmatism  and  most  myopic  in 
compound  myopic  astigmatism.  In  Fig.  61  all  the 
examples  show  direct  astigmatism.  If  the  conditions 
are  reversed,  it  is  called  "  inverse  astigmatism."*  When 
the  meridians  are  exactly  oblique — i.e.,  at  an  ang  e  of 
45°  or  135° — it  is  called  "  oblique  astigmatism." 

Symmetric  Astigmatism  is  when  the  axis  of  the  principal 

*  The  old  nomenclature  of  these  two  forms  was  "  astigma- 
tism according  to  (or  with)  the  rule  "  and  "  against  the  rule." 


120 


THE  REFRACTION   OF  THE  EYE 


meridian  in  each  eye  is  identical;  for  instance,  the 
meridian  of  greatest  curvature  is  vertical  in  both  eyes,  or 
is  15°  from  the  vertical  passing  down  and  in,  in  both 
eyes;  and  Asymmetric  Astigmatism  is  the  reverse. 


ninf>lp 


Hup  A  ■^h(j 


Compoii 


Simple  Mi^op  A&ti<j  Compound 


Fig.  61. 

V,  Rays  passing  through  the  vertical  meridian;  H,  rays  passing 
through  the  horizontal  meridian. 


Homonymous  Astigmatism  is  when  the  axes  of  the 
principal  meridians  in  each  eye  are  more  or  less  parallel ; 
for  instance,  the  axis  of  the  correcting  cylinder  passes 
down  and  in  15°  from  the  vertical  in  the  right  eye  and 
lo***  15°,  or  20°  down  and  out  in  the  left  eye. 


ASTIGMATISM  121 

The  Seat  of  Astigmatism. — In  regular  astigmatism 
the  seat  is  chiefly  in  the  cornea,  due  (i)  to  congenital 
malformation  of  the  cornea,  often  traced  to  heredity ;  or 

(2)  to  acquired  alteration  in  the  curves  of  the  cornea, 
produced  by  operations,  such  as  iridectomy  and  opera- 
tions for  cataract,  or  inflammation  of  the  cornea;  or 

(3)  to  pressure  from  tumours  in  the  lid. 

Transient  astigmatism  can  be  produced  by  pressure  on 
the  eye  with  the  finger,  or  by  contraction  of  the  lids  or 
the  extra-ocular  muscles.  Congenital  corneal  astigma- 
tism is,  more  or  less,  stationary  through  life;  acquired 
astigmatism  of  the  cornea  alters,  and  very  often  is  con- 
siderably reduced  by  time. 

Even  in  the  normal  eye  there  is  a  certain  amount  of 
astigmatism,  but  this  "  physiological  "  astigmatism  is  so 
small  that  it  can,  in  most  cases,  be  ignored. 

The  lens  may  also  be  the  seat  of  astigmatism,  which 
may  be  "  static  "  or  "  dynamic." 

The  static  lenticular  astigmatism  is  generally  small  in 
amount,  and,  being  in  the  same  meridian,  adds  itself  to 
that  of  the  cornea,  thus  increasing  the  total  astigmatism 
of  the  eye ;  but  sometimes  this  lenticular  astigmatism  is 
the  reverse  of  that  of  the  cornea,  and  so  corrects  it. 

Dynamic  Lenticular  Astigmatism  is  nearly  always  cor- 
rective, and  is  the  opposite  of  that  of  the  cornea.  It  is 
produced  by  an  unequal  contraction  of  the  ciliary  muscle, 
and  is  a  most  potent  factor  in  causing  eyestrain. 

Symptoms  of  Astigmatism. — When  the  astigmatism  is 
pronounced,  acuteness  of  vision  is  below  the  normal. 
Spherical  glasses  may  improve  the  distant  sight  to  a 
certain  extent,  but  the  correction  is  never  complete.  On 
directing  the  patient  to  look  with  one  eye  at  the  "  Fan  " 
(Fig.  62),  placed  5  or  6  metres  off,  or  nearer  if  necessary, 
we  find  that  he  can  see  certain  lines  more  distinctly  than 
others.  The  vertical  lines  may  be  seen  quite  black  and 
distinct,  the  horizontal  lines  being  faint,  or  vice  versa  ;  or 
the  oblique  lines  on  one  side  may  be  distinct,  those  on 


122 


THE  REFRACTION   OF  THE   EYE 


the  other  side,  at  right  angles  to  the  former,  being  in- 
distinct. If  all  the  radiating  lines  are  indistinct,  we  must 
make  one  of  the  meridians  emmetropic,  by  placing  before 
the  eye  the  weakest  concave  or  strongest  convex  spherical 
glass  that  is  required  to  make  one  set  of  lines  distinct  and 
black. 

As  we  have  already  seen,  when  rays  coming  from  a 
point  are  refracted  at  an  astigmatic  surface,  a  linear 
image  of  the  point  is  formed  at  the  focus  of  each  principal 
meridian,  and  the  direction  of  the  linear  image  is  at  right 
angles  to  the  meridian  at  whose  focus  it  is  formed.  Thus, 
when  a  patient  sees  the  horizontal  lines  distinctly  and  the 


lOO         90         QQ 


Fig.  62. 


lines  as  they  pass  to  the  vertical  become  less  distinct, 
reaching  the  maximum  of  indistinctness  in  the  vertical 
lines,  we  know  that  the  vertical  meridian  is  emmetropic, 
or  nearly  so.  Such  a  patient  would  complain  that  the 
letters  of  the  test  type  were  spread  out  horizontally, 
and  if  we  place  before  the  eye  a  stenopaic  disc  with  the 
si  t  vertical,  we  shall  find  the  phenomena  of  astigmatism 
disappear ;  he  sees  all  the  lines  with  equal  clearness,  and 
the  letters  appear  normal,  because  the  vertical  slit  has 
cut  off  all  the  horizontal  rays  that  caused  the  blurring. 
Astigmatic  Headache. — Eyestrain  is  the  commonest 
symptom  of  astigmatism,  and  of  all  the  forms  of  eye- 


ASTIGMATISM  123 

strain,  headache  is  by  a  long  way  the  commonest.    The 
strain  is  produced  in  two  ways : 

1 .  When  the  astigmatism  is  pronounced,  the  eye  has  to 
accommodate  in  order  to  obtain  clearer  images. 

Sturm  asserted  that  accommodation  was  not  made  for 
either  of  the  linear  foci,  but  for  a  point  between  the  two 
where  the  image  is  approximately  a  circle ;  but  Javal,  in 
his  later  researches,  found  that  it  was  the  vertical  focal 
line  that  was  sought  after. 

Tscherning  points  out  that  among  the  reasons  for  this 
preference  is  the  fact  that  in  reading,  the  legibility  of  the 
letters  depends  especially  on  the  distinctness  with  which 
the  vertical  lines  are  seen.  We  can  prove  this  for  our- 
selves by  holding  up  before  one  eye  (the  other  being 
closed)  a  concave  cylinder  of,  say,  i»5  d,  with  its  axis 
vertical;  if  we  turn  the  axis  round  to  the  horizontal, 
making  ourselves  vertically  hyperopic,  the  letters  are 
spread  out  vertically,  and  the  words  are  very  much 
clearer. 

2.  Meridional  Asymmetrical  Accommodation. — When 
the  astigmatism  is  small,  the  error  can  be  corrected  by  an 
unequal  contraction  of  the  ciliary  muscle,  producing  an 
astigmatism  of  the  lens  the  opposite  of  that  of  the  cornea. 

With  few  exceptions,  the  seat  of  regular  astigmatism  is 
in  the  cornea,  due  to  a  difference  in  the  curvature  of  the 
different  meridians;  added  to  this  there  is  sometimes 
found  a  "  static  lenticular  astigmatism,"  due  to  a  differ- 
ence in  the  curvature  of  the  different  meridians  of  the 
lens,  and  the  two  together  make  up  the  total  astigmatism 
of  the  eye  which  is  revealed  under  an  ordinary  examina- 
tion. But  most  frequently,  although  astigmatism  of  the 
eye  is  suspected,  where  it  is  of  low  degree  it  may  be  im- 
possible to  detect  it  without  resorting  to  a  cycloplegic. 
Bonders  in  1864  first  drew  attention  to  this,  and  he 
pointed  out  that  the  corneal  astigmatism,  when  smill  in 
amount,  was  corrected  by  an  astigmatism  of  the  lens  ; 
thus,  if  there  was  direct  astigmatism  of  the  cornea  of  •25, 


124  THE   REFRACTION   OF  THE   EYE 

there  would  be  inverse  astigmatism  of  the  lens  of  the 
same  amount,  neutralizing  and  masking  the  defect. 
Dobrowolsky  in  1868  asserted  that  this  lenticular 
astigmatism  was  produced  by  an  unequal  contraction  of 
the  ciliary  muscle ;  and  Hensen  and  Voelckers,  later, 
have  shown  by  experiments  upon  animals  that  this 
unequal  contraction  is  possible.  They  showed  that 
when  a  filament  of  the  ciliary  nerve  was  divided,  the 
portion  of  the  muscle  supplied  by  it  was  relaxed,  and  that 
on  stimulating  the  cut  end  a  local  contraction  took  place. 

But,  in  addition  to  this  physiological  proof,  the 
clinical  proofs  are  even  more  conclusive. 

Let  us  take  a  typical  case.  A  patient  complains  of 
headache,  accentuated  by  near  work.  Examination  re- 
veals no  refractive  error.  The  ciliary  muscle  is  paralyzed, 
and  astigmatism  is  discovered.  This  is  corrected  by 
cylinders,  the  glasses  are  ordered  to  be  worn  always, 
and  in  a  short  time  the  headache  disappears. 

Again,  very  often  when  the  effect  of  the  cycloplegic 
has  passed  off,  the  patient  refuses  the  cylinder  that  im- 
proved his  vision  under  atropine.  He  tells  you  that  it 
makes  his  vision  worse.  In  spite  of  this  you  prescribe  it, 
and — this  is  a  very  important  point — you  insist  on  the 
glasses  being  worn  always.  He  returns  in  a  month  or 
two,  assuring  you  that  his  headache  has  entirely  dis- 
appeared, that  he  has  become  accustomed  to  the  glasses, 
but  that  he  cannot  now  see  as  well  without  them  as  he 
could  before  using  them. 

What  has  happened  ?  At  first,  when  the  effect  of  the 
atropine  has  passed  off,  the  ciliary  muscle  returns  to  its 
old  habit  of  unequal  contraction,  and  consequently  the 
correcting  glasses,  instead  of  helping,  make  matters 
Worse ;  but  by  constantly  wearing  them  the  necessity  for 
this  unequal  contraction  disappears,  the  muscle  resumes 
the  normal  condition,  and  allows  the  glasses  to  do  the 
work.  Vision  is  apparently  worse  without  the  glasses, 
because  the  muscle  has  forgotten  its  bad  habit;  but,  of 


ASTIGMATISM  125 

course,  like  all  bad  habits,  it  can  be  easily  re- acquired. 
The  patient  has  lost  nothing  but  his  headache.  What 
stronger  proofs  could  there  be  that  this  unequal  con- 
traction does  occur  ? 

Further,  as  lenticular  astigmatism  must  necessarily  he 
very  small,  probably  rarely  higher  than  *$,  it  can  only 
neutralize  a  low  degree  of  astigmatism  in  the  cornea,  and  it 
is  in  these  cases  where  headache  is  most  frequent. 

It  is  easy  to  understand  how  this  unequal  contraction 
of  the  ciliary  muscle  causes  discomfort  or  pain,  especially 
in  a  neurotic  subject.  It  may  be  also  that  several  causes 
are  present,  such  as  constipation,  worry,  etc.,  and  that 
this  form  of  eyestrain  is  the  "  last  straw  on  the  camel's 
back."  Sometimes  it  is  only  towards  middle  age,  when 
the  accommodative  power  is  lessened,  or  the  nerve 
energy  lowered,  that  the  strain  shows  itself. 

This  unequal  contraction  of  the  ciliary  muscle  must 
interfere  with  the  nutrition  of  the  lens,  and  it  is  most 
likely  a  very  potent  factor  in  the  causation  of  cataract. 

In  high  astigmatism  there  may  be  some  asymmetry  of 
the  face,  but  otherwise  there  are  no  physical  appearances 
that  indicate  astigmatism.  In  oblique  astigmatism  the 
patient  may  acquire  the  habit  of  holding  the  head  on  one 
side,  but  this  is  not  always  the  case ;  on  the  other  hand, 
if  a  patient,  in  reading  the  distant  types,  does  hold  the 
head  obliquely,  we  may  be  almost  certain  that  oblique 
astigmatism  is  present. 

Diagnosis  and  Measurement  of  Astigmatism. — ^There 
are  numerous  methods  for  detecting  and  measuring 
astigmatism,  and  they  may  be  ranged  under  two  heads : 

I.  Objective  Methods — 

(a)  The  Shadow  Test,  or  Rhinoscopy  (see  page  74). — 
The  patient  being,  if  possible,  under  the  influence  of  a 
cycloplegic,  the  refraction  of  the  different  meridians  is 
estimated  in  the  manner  described  on  page  78.  When 
all  the  meridiansjhave  the  same  refraction,  there  is  no 
astigmatism;  when  there  is  a  difference,  astigmatism  is 


126  THE    REFRACTION  OF  THE  EYE 

present,  and  its  degreee  is  estimated  by  the  difference 
between  the  meridian  of  least  and  the  meridian  of 
greatest  refraction.  The  axis  of  the  correcting  cyHnder 
will  be  in  the  same  direction  as  that  of  the  meridian  of 
least  refraction  (see  Shadow-test,  page  yS).  For  in- 
stance, supposing  we  find  the  vertical  meridian  shows 
a  hyperopia  of  1-5  and  the  horizontal  a  hyperopia  of  2 '5, 
then  the  astigmatism  equals  i,  and  the  axis  of  the  convex 
cylinder  is  vertical.  Or,  again,  using  a  concave  mirror, 
supposing  we  find  the  shadow  moves  "  against "  in  the 
direction  15°  from  the  vertical  down  and  in  and  a  +2 
corrects,  and  the  meridian  at  right  angles  gives  a  shadow 
"  with  "  corrected  by  - 1,  we  have  mixed  astigmatism 
amounting  to  3  d  and  corrected  by  a  concave  cylinder 
-3  placed  15°  from  the  vertical  down  and  in  and  a  +1 
sphere,  or  a  convex  cylinder  +3  with  its  axis  15°  from 
horizontal  down  and  out  and  a  -  2  sphere. 

Retinoscopy  is  a  very  valuable  help  in  estimating 
astigmatism;  it  is  accurate  and  simple,  but  is  not  so 
delicate  as  the  ophthalmometer. 

(6)  The  Ophthalmometer. — The  instrument  first  suggested  by 
Helmholtz,  and  improved  by  Javal  and  Schiotz,  has  been  made  in 
many  forms,  but  perhaps  the  model  made  by  Meyrowitz  is  the 
best  (Figs.  63  and  64). 

The  ophthalmometer  measures  the  curvatures  of  the  cornea, 
and  thus  enables  us  to  ascertain  the  presence  of  astigmatism,  and, 
if  present,  its  amount,  the  direction  of  the  principal  meridians, 
and  the  character — i.e.,  whether  we  are  dealing  with  direct, 
inverse,  or  oblique  astigmatism — and  all  this  information  is 
acquired  in  a  very  short  space  of  time  by  the  expert.  Moreover, 
the  patient  is  passive,  the  examination  being  purely  objective, 
and,  as  such,  of  immense  value  in  checking  the  subsequent 
subjective  test. 

It  is  true  that  the  ophthalmometer  only  gives  information 
concerning  the  astigmatism  of  the  cornea,  and  not  the  total 
astigmatism  of  the  eye,  but  this  is  exactly  the  information  we 
want;  for,  as  we  have  seen  (page  121),  the  lenticular  astigmatism 
when  dynamic  is  a  corrective  astigmatism,  and  disappears  under 
a  cycloplegic.  It  naturally  follows  that,  useful  as  this  instru- 
ment is  in  every  case,  in  those  cases  where  for  some  reason  a 
cycloplegic  is  prohibited,  the  ophthalmometer  is  exceptionally 
valuable,  and  in  the  diagnosis  and  estimation  of  low  errors  of 
astigmatism  it  is  indispensable. 


THE   OPHTHALMOMETER  12  7 

The  essential  parts  of  the  instrument  are  two  mires,  whose 
images  are  reflected  on  the  cornea  of  the  patient,  and  which  are 
seen  by  the  observer  through  a  telescope  containing  a  double 
prism  between  two  bi-convex  lenses.  The  patient's  chin  rests 
on  a  support,  and  his  forehead  should  press  against  the  top  of  the 
stand  to  insure  perfect  rest,  and  the  eye  not  being  examined  is 
covered  by  a  sliding  clip.  The  mires  are  carried  on  an  arc 
which  can  be  rotated  into  any  position,  and  there  is  a  graduated 


Fig.  63. 


disc  on  the  observer's  side  of  the  instrument,  which,  by  means  of 
a  pointer,  shows  the  meridian  of  the  arc  (Fig.  64) . 

The  two  mires  are  made  of  porcelain,  and  illuminated  by  elec- 
tric lamps  behind  them;  they  are  operated  by  means  of  a  gear 
movement,  and  are  thus  made  to  approach  or  separate  from  each 
other,  their  position  being  indicated  by  pointers  working  on  a 
disc  on  the  observer's  side  (Fig.  64) . 

When  electricity  is  not  available,  gas  or  lamps  must  be  used  at 


128 


THE   REFRACTION   OF  THE   EYE 


the  side  of  the  patient's  head  and  reflected  on  to  the  mires,  as 
ordinary  daylight  is  an  insufficient  illuminant.^,^j^ 


Fig.  64. — ^Latest   Model   of   Meyrowitz   Ophthalmometer 
(Observer's  Side). 

Seated  on  the  other  side  of  the  stand,  the  surgeon  looks  through 
the  eyepiece  and  points  the  telescope  to  the  eye,  and,  by  means  of 


THE  OPHTHALMOMETER  12^ 

a  rack  and  pinion  on  the  upright,  moves  it  up  or  down  until  he 
sees  four  figures  on  the  patient's  cornea,  which  are  two  reflections 
of  each  of  the  mires.*  Ignoring  the  outside  figures,  he  now 
accurately  focuses  the  two  inside  ones  by  means  of  a  rack  and 
pinion. 

The  first  step  is  to  ascertain  the  axis  of  the  astigmatism  when 
present,  and  we  start  with  the  arc  horizontal,  and  note  whether, 
in  the  reflection  of  the  two  mires,  the  deep  black  line  which  runs 
through  the  centre  is  a  continuous  black  line  running  through 
both;  if  not,  the  arc  must  be  revolved  to  the  right  or  left  (but 
never  more  than  45°)  until  this  result  is  obtained;  we  then  read 
off  the  axis  on  the  dial.  The  next  step  is  to  so  adjust  the  mires 
that  their  reflections  are  just  in  contact,  as  in  Fig.  65,  a.  This, 
then,  is  the  primary  position  of  the  ophthalmometer — ^viz.,  with 
the  central  black  lines  of  the  two  mires  forming  an  unbroken 
black  line,  and  the  two  inner  edges  of  the  mires  just  in  contact. 

We  now  revolve  the  arc  through  a  complete  right  angle,  and 
if  the  relative  position  of  the  reflected  mires  has  not  changed, 
then  there  is  no  corneal  astigmatism ;  but  if,  with  the  arc  vertical 


Fig.  65. 

or  nearly  so,  the  reflections  overlap  (as  in  Fig.  65,  b),  there  is 
direct  astigmatism,  each  step  of  the  reflected  mire  overlapping 
representing  one  dioptre  of  astigmatism;  or,  to  be  more  correct, 
we  note  the  exact  position  of  the  underneath  pointer  on  the  disc, 
bring  the  superficial  pointer  exactly  over  it,  then  readjust  the 
mires  so  that  they  are  again  in  contact.  If  astigmatism  is  present, 
the  two  pointers  are  separated  by  an  interval  which  represents 
the  amount  of  the  astigmatism,  which  amount  is  indicated  by 
the  divisions  on  the  disc. 

If,  on  revolving  the  arc  from  the  horizontal  to  the  vertical 
position,  the  reflections  separate,  we  are  dealing  with  inverse 
astigmatism,  and  we  must  then  make  the  secondary  position  our 
primary  position,  and  proceed  as  before. 

If  used  as  a  servant,  and  not  allowed  to  become  master,  the 
ophthalmometer  is  one  of  the  most  valuable  adjuncts  to  the 
ophthalmologist's  consulting-room,  for  after  some  practice,  and 

*  The  telescope  has  an  adjustable  eyepiece  at  the  surgeon's 
end,  and  in  the  centre  an  achromatic  objective,  which  has 
between  its  two  bi-convex  lenses  a  bi-refringent  prism. 

9 


130  THE   REFRACTION   OF  THE   EYE 

when  thoroughly  mastered,  in  about  one  minute  the  observer 
ascertains  (i)  whether  astigmatism  is  present,  {2)  the  amount, 
and  (3)  the  direction  of  the  axis  of  the  principal  meridian;  and, 
moreover,  this  is  done  with  such  delicacy  that  one-eighth  of  a 
dioptre  of  astigmatism  is  revealed.  It  is  also  the  quickest  method 
of  diagnosing  irregular  astigmatism  (see  page  139). 

In  low  errors,  probably  owing  to  some  static  astigmatism  of  the 
lens,  the  total  astigmatism  of  the  eye  generally  shows  about  '25 
of  inverse  astigmatism,  in  addition  to  the  ophthalmometric 
measurement.  Thus,  when  the  ophthalmometer  shows  no 
astigmatism,  there  is  '25  inverse  astigmatism;  when  it  shows 
direct  astigmatism,  there  is  '25  less;  and  when  it  shows  inverse 
astigmatism,  there  is  generally  about  '25  more,  but  this  varies 
in  different  instruments;  the  particular  idiosyncrasy  of  any 
instrument  is  very  soon  discovered. 

The  following  points  should  be  very  carefully  observed,  in 
order  to  prevent  inaccurate  results: 

1.  Impress  upon  the  patient  the  imperative  necessity  of 
keeping  the  forehead  pressed  against  the  stand.  If  on  turning 
the  arc  into  the  secondary  position  the  mires  have  to  be  re- 
focused,  we  know  the  patient  has  moved.  Once  the  mires  are 
focused  in  the  primary  position,  the  focusing  must  not  be  altered. 

2.  It  is  equally  important  to  insist  upon  the  patient  looking  all 
the  time  into  the  centre  of  the  telescope;  otherwise  an  astigma- 
tism will  appear  which  is  not  a  central  astigmatism,  and  is  conse- 
quently not  the  astigmatism  we  wish  to  ascertain. 

Hardy's  ophthalmometer,  in  which  the  mires  are 
stationary  and  the  prisms  are  movable,  is  said  to  be 
a  more  accurate  instrument,  but  I  have  personally  failed 
to  confirm  this. 

The  Sutcliffe  keratometer,  which  is  a  one-position 
ophthalmometer,  is  said  to  be  a  very  reliable  instrument. 
A  full  description  will  be  found  in  the  Ophthalmoscope, 
April,  1909. 

(c)  The  Ophthalmoscope. — Ophthalmoscopically,  astig- 
matism is  revealed  by  observing  that  all  parts  of  the 
fundus  are  not  in  focus  at  the  same  time,  no  matter  what 
lens  we  turn  into  position. 

Estimation  of  the  Amount  of  Astigmatism  hy  the  Oph- 
thalmoscope.— Bearing  in  mind  the  fact  that  vertical 
vessels  are  seen  through  the  horizontal  meridian,  and 
horizontal  vessels  through  the  vertical  meridian,  we 
focus,  for  instance,  the  vessels  passing  horizontally  from 
the  disc  to  the  macula,  and  find  the  weakest  concave  or 


ASTIGMATISM  13 1 

strongest  convex  glass  that  gives  us  the  best  picture; 
this  will,  of  course,  give  us  the  refraction  of  the  meridian 
at  right  angles  to  this— viz.,  the  vertical.  We  then  focus 
the  vessels  that  pass  up  or  down  from  the  disc,  and 
estimate  thus  the  refraction  of  the  horizontal  meridian, 
and  the  difference  between  the  two  meridians  is  the 
measure  of  astigmatism. 

When  the  principal  meridians  are  not  horizontal  and 
vertical,  we  focus  vessels  passing  obliquely — say  up  and 
out  from  the  disc — and  afterwards  those  passing  down 
and  out,  and  so  on. 

The  patient  must  be  under  a  cycloplegic,  and  the 
observer's  accommodation  must  be  relaxed,  which  intro- 
duces the  personal  element  and  causes  this  method  to  be 
rarely  used  by  ophthalmologists  in  preference  to  retin- 
oscopy  or  the  ophthalmometer. 

It  is  important  to  remember  that  the  vessels  to  be 
observed  should  be  those  situated  near  the  macula, 
because  we  are  estimating  the  refraction  of  the  central 
part  of  the  dioptric  system. 

It  is  hardly  necessary  to  add  that  this  method  is  not 
delicate  enough  for  the  diagnosis  of  small  errors  of 
astigmatism. 

The  Ophthalmoscope  by  the  Indirect  Method. — This  is 
of  little  use  in  diagnosing  low  errors.  When  the  error 
is  pronounced,  the  optic  disc  appears  oval,  and  its 
elongation  is  in  the  meridian  of  least  curvature.  When 
the  focus-glass  is  withdrawn  from  the  eye,  if  the  aerial 
image  remain  the  same  size  in  one  meridian  but  become 
smaller  in  the  other,  the  case  is  one  of  simple  hyperopic 
astigmatism;  and  if  the  image  become  larger,  it  is 
a  case  of  simple  myopic  astigmatism.  In  compound 
hyperopic  astigmatism  the  image  becomes  smaller  in 
both  meridians,  but  more  so  in  one ;  and  the  reverse,  of 
course,  in  compound  myopic  astigmatism.  In  mixed 
astigmatism,  on  withdrawing  the  focus-glass,  the  disc 
appears  to  become  relatively  larger  in  the  direction  of 


132 


THE  REFRACTION  OF  THE  EYE 


the  maximum,  and  relatively  smaller  in  the  direction  of 
the  minimum  meridian. 

3.  Subjective  Methods.— (^)  Methods  based  on  the 
fact  that  when  astigmatism  is  present  lines  running  in 


different  directions  are  not  all  clearly  seen  at  the  same 

Clock-face. 

The  fan,  or  "  rising  sun." 

"  Confusion  letters,"  such  as  E  and  Z. 

Pray's  letters. 

The  clock-face  (Fig.  66)  SLud  fan  (Fig.  62),  if  observed 
by  a  non-astigmatic  eye  at  rest,  will  be  seen  to  have  the 
lines  all  equally  black;  but  in  astigmatism  the  vertical 
lines,  for  instance,  will  appear  black,  while  the  horizontal 
are  gray,  or  the  oblique  down  and  in,  black,  and  the 
oblique  down  and  out,  gray. 


ASTIGMATISM  133 

It  should  be  borne  in  mind  that  the  meridian  of  the 
eye  which  corresponds  to  the  darkest  Hnes  is  the  meridian 
of  greatest  ametropia;  thus,  a  patient  who  requires 
- 1  cyl.  axis  horizontal  to  make  all  the  lines  appear 
equally  dark  has  simple  myopic  direct  astigmatism,  and 
sees  the  vertical  lines  darkest  before  correction.  The 
patient's  eyes  must  be  under  a  cycloplegic,  and  concave 
or  convex  spherical  glasses  may  have  to  be  placed  in 
front  of  the  eye  to  correct  any  general  ametropia  present, 
otherwise  the  whole  chart  may  be  out  of  focus. 

This  method  is  not  delicate  enough  for  very  low 
degrees  of  astigmatism,  and,  in  fact,  is  rarely  used. 

Confusion  Letters. — There  are  certain  letters  which 
astigmatics  often  confuse,  such  as  D  and  O  or  U,  E  and 
Z,  S  and  B,  and  when  a  patient  on  using  Snellen's  types 
makes  these  mistakes  we  suspect  astigmatism. 

Fray's  Letters  are  letters  printed  v^th  stripes  running 
in  different  directions ;  the  patient  selects  the  letters  that 
appear  darker  than  the  others,  and  the  direction  of  the 
stripes  in  the  selected  letter  or  letters  corresponds  to  the 
meridian  of  greatest  ametropia  (Fig.  67) . 

(6)  The  Chromo-aberration  or  Cobalt-blue  Test,  based 
on  the  principle  that  violet  or  blue  rays,  being  more 
refrangible  than  red,  are  brought  to  a  focus  sooner 
(Chromatic  Aberration) . 

Cobalt-blue  glass  contains  a  great  deal  of  red,  and 
allows  only  blue  and  red  rays  to  pass.  Such  a  glass  of 
suitable  thickness  is  mounted  in  a  trial  frame  placed 
before  the  eye  to  be  examined,  the  other  eye  being 
excluded.  A  clear  round  point  of  light  should  be  looked 
at  from  a  distance  of  4  to  6  metres.  When  the  eye  is 
emmetropic,  the  light  appears  violet;  when  hyperopic, 
the  light  appears  blue  in  the  centre,  surrounded  by  a  red 
ring ;  and  when  myopic,  red  in  the  centre,  surrounded  by 
a  blue  ring.  If  astigmatism  be  present,  the  light  appears 
oblong,  vertically,  or  horizontally,  in  different  charac- 
teristic shapes.     Scientifically,  this  is  a  most  interesting 


134  THE  REFRACTION  OF  tHE  EYE 

test,  but  its  weakness  consists  in  the  fact  that  it  is  purely 
subjective,  and  that  the  surgeon  is  entirely  dependent  on 
the  patient's  description, 
(c)  Examination  of  the  Patient  before  Snellen*s  Types— 

(i)  With  the  Stenopaic  Slit  (Fig.  58). — This  is  an  opaque 
disc  to  fit  into  the  trial  frame,  and  through  the  centre 
there  is  a  slit  about  i  milHmetre  broad.     On  looking 


/6S' 


W  B 


go"  /OS'  fS° 


1iili.11   i/ 


US' 


<55^ 


rs 


Fig.  67. 

through  this  sht  the  patient  sees  only  rays  passing 
through  the  meridian  corresponding  to  the  slit ;  all  other 
rays  are  excluded,  and  the  glass  in  front  of  this  slit 
that  gives  the  best  vision  represents  the  refraction  of 
this  meridian.  The  slit  is  then  turned  round  at  right 
angles,  and  the  refraction  of  the  other  meridian  is  taken. 
The  difference  between  the  two  meridians  is  the  amount 


ASTIGMATISM  135 

of  astigmatism,  and  the  value  of  the  cylinder  that  will 
have  to  be  employed  to  correct  the  defect. 

For  instance,  when,  with  the  slit  vertical,  |  is  read, 
and  convex  glasses  make  vision  worse,  this  meridian  is 
emmetropic.  On  turning  the  slit  round,  when  a  +i 
glass  is  required  to  get  ^,  this  meridian  is  hyperopic, 
the  astigmatism  is  i  d,  and  is  corrected  by  a  cylinder 
+  1  axis  vertical. 

This  method  is  not  very  accurate,  as  much  depends 
on  the  width  of  the  slit,  and  better  and  quicker  methods 
have  superseded  it ;  but  sometimes  in  a  difficult  case  of 
mixed  astigmatism  it  is  of  assistance. 

(2)  With  Cylinders. — This  method  is  too  wearisome  to 
be  used  by  itself,  but  when  we  have  ascertained  the 
refraction  by  the  ophthalmometer  and  retinoscopy,  we 
always  employ  it  as  a  final  test. 

Treatment. — All  those  engaged  in  refraction  work 
should  observe  two  golden  rules: 

First  Rule :  Always  suspect  the  presence  of  astig- 
matism. 
Second  Rule :  Never  be  satisfied  that  astigmatism 
is  eliminated  unless  the  examination  has  been 
made  under  a  cycloplegic  in  all  under  forty  or 
forty-five  years  of  age. 
There  is  no  refractive  error  in  which  cycloplegics  are  of 
such  paramount  importance  as  in  astigmatism  of  a  small 
amount.*     The  ciliary  muscle  has  formed  a  bad  habit 

*  The  following  table  shows  the  relative  frequency  of  small 
errors : 

500  consecutive  refractions  =  1000  eyes. 

•12  164 

'Under  '5 

Under  i  d     -[ 

Astigmatism       ■{  ['5  and  over 


•25  306    V  542 

•37  72    j 

•5  108   \ 


•62  14    V   192 

1-75  70  J 

I  D  and  over  .....       132 

No  astigmatism         ........       134 

Ophthalmometer  correct  =  982.  ^^^^ 


136  THE  REFRACTION   OF  THE   EYE 

(of  which  the  patient  is  often  quite  unconscious),  and 
only  gives  up  this  habit  when  forced  to  do  so  by  being 
paralyzed. 

Cylindrical  lenses  correct  regular  astigmatism  of  the 
cornea,  and  when  the  error  is  small,  do  the  work  that 
the  ciliary  muscle  has  been  doing  at  so  great  a  cost  to 
the  nervous  system.  When  the  error  is  large,  certainly 
when  it  is  over  '75  d,  the  ciliary  muscle  cannot  correct 
the  defect,  and  consequently  makes  no  attempt  to  do 
so ;  but  the  greatest  care  must  be  exercised  in  giving  the 
exact  cylinder  that  corrects  the  defect,  because,  if  a 
small  portion  is  left  uncorrected,  the  ciliary  muscle  can 
do  the  rest  of  the  work  and  strain  results.  For  instance, 
by  retinoscopy  we  find  an  eye  with  the  horizontal 
meridian  showing  +4,  and  the  vertical  +2.  We  find 
that  a  cylinder  +2,  axis  vertical,  and  a  sphere  +1,  gives 
f ,  and  when  the  effects  of  the  cycloplegic  have  passed 
off,  we  give,  say,  cylinder  +2  axis  vertical.  Had  we 
been  a  little  more  careful  we  should  have  found  that 
the  best  result  was  obtained  by  a  cylinder  +2*25  axis 
vertical,  and  this  '25  we  have  omitted  to  correct  is  cor- 
rected by  the  lens,  and  we  introduce  eyestrain  which 
did  not  exist  before. 

The  estimation  of  astigmatism  is  now  made  entirely 
by  (i)  the  ophthalmometer,*  (2)  the  shadow  test,  and 
(3)  the  final  trial  of  glasses  before  Snellen's  types  placed 
at  6  metres  from  the  patient. 

Armed  with  the  knowledge  of  the  refraction  of  the 
principal  meridians  and  the  direction  of  the  axes,  the 
final  test  is  very  easy.  The  difference  between  the  two 
meridians  represents  the  strength  of  the  cylinder,  and 
the  spherical  glass  is  represented  by  the  refraction  of 
the  weakest  meridian.  Thus,  when  the  horizontal 
meridian  is  -  4,  and  the  vertical  -  6,  we  take  a  cylinder 
-  2,  place  its  axis  to  correspond  with  the  least  ame- 

*  This  instrument  is  of  the  utmost  value  in  correcting  small 
errors;  it  was  correct  in  982  cases  out  of  1,000  (see  table,  page  135). 


ASTIGMATISM  137 

tropic  meridian — that  is,  horizontal — and  combine  it 
with  a  spherical  glass  -  4. 

In  mixed  astigmatism  the  selection  of  the  glasses  is  a 
little  more  complicated. 

As  mentioned  before,  the  difference  in  refraction  of 
the  two  meridians  equals  the  amount  of  astigmatism. 
When  the  horizontal  is  +3,  and  the  vertical  -2,  5  d 
is  the  amount  of  the  astigmatism.  Now,  we  can  use 
either  a  concave  cylinder  and  convex  sphere  or  vice 
versa,  and  sometimes  it  is  as  well  to  try  both  kinds,  as 
one  may  be  preferred.  Let  us  in  this  case  take  a  -  5 
cylinder,  and  place  it  in  the  trial  frames  with  the  axis 
horizontal  {i.e.,  at  right  angles  to  the  myopic  meridian), 
and  correct  the  hyperopia  with  a  +3  sphere.  When 
the  patient  recovers  from  the  cycloplegic,  this  sphere 
will  have  to  be  reduced  to  +2,  or  even  +i*5.  As  a 
general  rule  it  is  best  to  choose  the  cylinder  that  corrects 
the  most  defective  meridian.  For  instance,  the  vertical 
meridian  is  - 1,  and  the  horizontal  +4.  Here  select 
a  +5  cylinder,  set  vertically,  combined  with  a  -i 
sphere,  which  may  have  to  be  strengthened  to  - 1*5 
when  the  cycloplegic  has  passed  off. 

The  optician  should  be  instructed  to  set  the  concave 
glass  next  the  eye,  in  order  to  obtain  a  periscopic  effect. 

The  rule  in  mixed  astigmatism  is:  The  cylinder 
represents  the  difference  between  the  two  meridians, 
with  its  axis  at  right  angles  to  the  meridian  whose  sign 
it  corresponds  to,  and  the  spherical  glass  should  be  the 
value  of  the  meridian  whose  sign  is  the  opposite  to  the 
cylinder. 

In  testing  the  patient  under  a  cycloplegic  6  metres 
from  Snellen's  type  with  the  trial  glasses,  we  may  have 
to  add  or  subtract  from  the  various  glasses,  sphericals 
or  cylindricals,  to  get  the  best  result,  and  the  best 
position  of  the  axis  of  the  cylinder  may  be  a  few  degrees 
different  from  what  the  retinoscopy  or  the  ophthal- 
mometer indicated;  but  we  must  remember  that  the 


138  THE   REFRACTION   OF  THE   EYE 

subjective  examination  must  always  have  the  "  last 
word,"  the  objective  examination  being  our  guide,  and 
never  our  master. 

Another  important  rule  to  be  observed  in  the  treat- 
ment of  astigmatism  is  that  it  should  be  fully  corrected, 
and  when  the  patient  has  recovered  from  the  cycloplegic 
neither  the  power  nor  the  axis  of  the  cylinder  should  be 
altered,  although  we  may  have  to  deduct  from  or  add 
to  the  spherical  lens.  The  exceptions  to  this  rule  are 
few,  and  are  as  follows :  (i)  In  children  under  seven  years 
of  age,  when  the  astigmatism  is  less  than  '5,  no  cylinder 
need  be  ordered  if  no  symptom  of  eyestrain  be  present ; 
(2)  in  patients  over,  say,  40  years  of  age,  who  have  never 
had  their  astigmatism  corrected  and  who  require  a  high 
cylinder,  as  this  latter  may  give  discomfort  at  first,  a 


10 


^ 


^30 
Fig.  68. 

slight  reduction  may  be  made  in  the  power,  but  the  full 
correction  should  be  given  as  soon  as  possible  later. 

In  examining  patients  for  astigmatism,  atropine 
should  be  previously  used  up  to  the  age  of  25,  and  even 
up  to  the  age  of  35,  if  a  low  error  is  suspected;  hom- 
atropine  is  sufficient  for  older  patients;  after  50  no 
cycloplegic  is  necessary. 

In  ordering  cylinders  be  careful  to  indicate  accurately 
the  axis.  If  this  is  done  by  simply  writing  down  the 
degrees,  a  mistake  may  occur,  as  there  is  no  uniformity 
at  present  in  the  numbering  of  the  trial  frames  or  pre- 
scription forms.  The  International  Ophthalmic  Con- 
gress at  Naples,  1909,  suggested  that  the  nasal  ex- 
tremities of  the  horizontal  line  should  be  zero,  the 
temporal  extremities  180°,  and  the  vertical  line  90°,  but 
this  has  not  yet  been  universally  adopted.    The  simplest 


IRREGULAR   ASTIGMATISM 


139 


method  is  to  indicate  the  axis  in  degrees  from  the 
vertical  or  the  horizontal,  as  in  Fig.  68,  or  on  an  optician's 
form,  or,  better  still,  on  a  form  engraved  or  stamped  on 
the  paper,  as  in  Fig.  69. 

Note. — So  important  is  it,  when  treating  small  errors 
of  astigmatism,  to  correct  even  the  smallest  amount, 
that  the  trial  case  should  be  fitted  with  cylinders  repre- 
senting fractions  of  •12  up  to  i  d — i.e.,  in  addition  to 
•25,  '50,  '75,  there  should  also  be  •12,  'Z7>  '62,  and  '87 
cylinders. 

Irregular  Astigmatism. — Physiological  irregular  astig- 
matism is  present  in  all  lenses.  It  is  due  to  separate 
sectors  of  the  lens  having  a  different  refractive  power, 
and  is  infinitesimal  in  amount.     It  is  this  condition 


Fig.  69. 

which  causes  a  bright  star  to  appear  as  a  radiating 
figure  instead  of  a  bright  point. 

Sometimes  the  refractive  power  of  the  separate 
sectors  of  the  lens  is  so  great  that  several  images  of  a 
point  are  formed,  and  it  is  in  this  way  we  get  monocular 
polyopia  in  incipient  cataract. 

Irregular  Astigmatism  in  the  Cornea  may  be  consider- 
able, and  is  generally  the  result  of  disease,  such  as  ulcers, 
nebulae,  wounds,  and  conical  cornea.  It  is  due  to  a 
difference  in  curvature  in  different  parts  of  the  same 
meridian,  and  often  produces  distortion  of  objects,  which 
regular  astigmatism  rarely  does. 

Irregular  astigmatism  is  diagnosed  by — 

I.  The  Ophthalmometer. — ^This  is  the  readiest  and  the 
easiest  method.    The  reflection  of  one  or  both  mires  is 


140  THE  REFRACTION   OF  THE  EYE 

distorted,  and  this  distortion  and  the  relative  positions 
of  the  mires  vary  irregularly  in  different  positions-  of 
the  arc. 

2.  Retinoscopy. — There  is  no  definite  shadow,  or,  if  it 
be  present,  it  behaves  in  an  irregular  manner,  and  glasses 
produce  no  definite  and  regular  effect. 

3.  Placido's  Disc  (Fig.  70). — This  is  a  round  disc 
supported  by  a  handle.     The  disc  is  about  7  or  8  inches 


in  diameter,  and  has  painted  on  one  side  alternate 
concentric  rings  of  black  and  white,  and  has  a  small  hole 
in  the  centre.  The  patient  stands  with  his  back  to  the 
light,  and  the  disc  is  held  by  the  surgeon  a  short  dis- 
tance from  the  patient's  eye  in  such  a  manner  that  an 
image  of  the  rings  is  thrown  on  to  the  cornea  to  be 
examined,  the  patient  being  directed  to  look  at  the 


IRREGULAR  ASTIGMATISM  I4I 

centre.  Looking  through  the  central  hole,  the  surgeon 
sees  a  diminutive  image  of  the  rings  on  the  cornea,  and 
if  the  latter  is  normal  the  rings  are  round  and  evenly 
separated.  In  regular  astigmatism  the  image  appears 
elliptical,  the  long  axis  corresponding  with  the  meridian 
of  least  curvature;  but  when  irregular  astigmatism  is 
present,  the  rings  are  "  crinkled  "  and  distorted. 

Treatment. — Place  before  the  eye  in  a  trial  frame  an 
opaque  disc  with  a  small  central  opening  (stenopaic 
disc),  shift  the  position  of  the  opening,  and  also  place 
up  different  lenses.  If  vision  be  improved,  prescribe 
stenopaic  spectacles,  with  the  lens  if  indicated;  and  if 
the  opening  be  eccentric,  specify  the  amount  of  de- 
centring. 

In  conical  cornea  this  treatment  is  sometimes  bene- 
ficial, but,  unfortunately,  in  most  cases  the  spectacle 
treatment  of  irregular  astigmatism  is  useless. 

As  regular  astigmatism  may  also  be  present,  it  is 
worth  while  making  the  above  examination  with  a 
stenopaic  slit,  turning  it  round  in  different  positions  to 
see  if  any  improvement  results  with  or  without  the  addi- 
tion of  a  convex  or  concave  spherical  lens;  and  if  any 
improvement  results  with  a  lens,  the  prescription  of  the 
equivalent  cylinder  may  improve  vision. 

If  the  defect  is  caused  by  a  corneal  nebula,  and 
especially  if  the  patient  is  young  and  the  opacity  not 
too  dense,  considerable  improvement  sometimes  follows 
the  use  of  the  yellow  oxide  of  mercury  ointment,  a  small 
piece  of  which  should  be  rubbed  into  the  cornea  by 
massage  through  the  upper  lid,  about  twice  a  week. 


CHAiPTER  IX 
PRESBYOPIA 

The  Influence  of  Age  upon  the  Accommodation. — In 

quite  early  youth  the  crystalUne  lens  is  practically  a 
small  bag  of  semifluid  jelly,  and  accommodation  takes 
place  by  its  being  squeezed  by  the  action  of  the  ciliary 
muscle  in  such  a  manner  that  its  antero-posterior 
diameter  is  enlarged  (see  page  31).  So  great  is  this 
"  squeezability  "  (if  I  may  use  the  term)  in  the  very 
young,  that  an  accommodative  power  of  20  d  can  often 
be  recorded.  As  age  advances,  a  hardening  process 
or  sclerosis  goes  on  in  the  lens  as  in  all  the  other  tissues 
of  the  body,  and  so  its  "  squeezability  "  becomes  less 
and  less  until  a  point  is  reached  when  the  near  point 
of  accommodation,  which  represents  the  fullest  accom- 
modative power,  has  so  far  receded  that  the  normal  eye 
requires  assistance  in  the  shape  of  a  convex  lens  in 
order  to  see  near  objects  distinctly.     This  is  presbyopia. 

At  the  age  of  10  years  the  average  emmetrope's  near 
point  is  7  cms.  from  the  eye,  and  his  far  point  being  at 
"  infinity,"  we  see  that  his  amplitude  of  accommodation 
is  14  D  (in  Fig.  71,  in  the  first  column  on  the  left,  there 
are  14  divisions  between  p  and  ;'),  whereas  at  the  age 
of  30  his  near  point  has  receded  to  14  cms.,  and  his 
amplitude  of  accommodation  is  then  only  7  D — -that  is, 
in  twenty  years  he  has  lost  half  of  his  accommodative 
power. 

The  same  happens  whatever  the  refractive  condition  of 
the  eye.    For  instance,  a  hyperope  of  4  d  (Fig.  72)  at 

142 


PRESBYdPtA 


143 


Near 
Point. 


Cms. 
7     •• 


fO     fS     20     25    30    3S 


Age. 
40    4S  SO    SS    €0    €S 


70    7S   80 


50 


f^ 

lA 

/J 

\, 

/? 

\ 

P 

// 

\ 

/O 

\ 

,9 

\ 

8 

\ 

7 

\, 

fi 

s. 

s 

\ 

V 

4- 

s 

J 

N 

? 

N 

*  t 

X 

0 

\ 

i^ 

:;>, 

p 

--- 

•  ( 

T 

-^ 

'? 

X 

-, 

X 

-^ 

-,f 

...J 

Fig. 


71- 


Showing  the  range  of  accommodation  of  an  emmetrope  at 
different  ages. 

the  age  of  10  has  his  near  point  10  cms.  from  the  eye, 
and  p  =  '-^;^  =  -W  =  iOD,r  is  negative,  and 

a  =  10  -  (  -  4) 
=  10  +  4 

=   14  D. 

Again,  at  30  we  see  (Fig.  72)  that  ^  =  3  d,  p  being 
now  33  cms.  from  the  eye,  and 

a  =  3  +  4 
=  7D. 

*  The  numerals  above  represent  years,  those  on  the  left 
dioptres.  The  line  p  p  represents  the  curve  of  the  punctum. 
proximum,  and  the  line  r  r  that  of  the  punctum  remotum. 


144  THE   REFRACTION   OF   THE   EYE 

70     IS     20     25     30    3S     4^    ^     SO    S6     60     66      70     76    80 


/I 

•-r 

m 

.9 

fi 

7 

V 

ff 

N 

,f 

\, 

4- 

\ 

,T 

\, 

? 

s. 

*/ 

s 

V 

0 

-- 

— 

--- 

... 

-- 

N 

^ 

-_. 

.__ 

--- 

--- 

-- 

.._. 

? 

'X 

,^ 

V. 

v,^ 

<7- 

s 

\, 

-^ 

r 

' 

>s 

P 

(tr 

r^ 

-^ 

7 

\ 

^ 

Fig.  72. 

Showing  the  range  of  accommodation  of  an  uncorrected  hyperope 
of  4  D  at  different  ages. 

A  myope,  say,  of  3  d  (Fig.  73)  has  his  near  point,  at 
the  age  of  10,  6  cms.  from  the  eye,  and  ^  =  17  d. 

a  =  17  -  3 

=   14  D. 

^  At  the  age  of  30  we  see  by  the  diagram  that  p  =  10  D, 
for  p  =  10  cms.,  and  R  is  still  33  cms.  on  the  positive 
side;  hence 

a  =  p  -  r 

=  10-3 

=  7D. 

\\'hatever  the  static  refraction  of  the  eye,  r  remains 
stationary  till  about  the  age  of  55,  when  we  see  that  in 


PRESBYOPIA 


145 


all  three  diagrams  it  begins  to  curve  downwards,  show- 
ing that  the  emmetrope  becomes  hyperopic,  the  hyper- 
ope  more  so,  and  the  myope  less  so.  This  is  called 
acquired  hyperopia.  A  point  is  finally  reached  when  p 
and  r  unite — in  other  words,  when  all  accommodation 
ceases ;  this  is  about  the  age  of  75 ;  but  in  emmetropia  and 

Age. 


/O    /S     20    26     30    35     <^     ^i     SO    SS    €0    €S    70     7S  80 


:i 


^: 


::x 


:5: 


Fig.  73. 


Showing  the  range  of  accommodation  of  an  uncorrected  myope 
of  3  D  at  different  ages. 


hyperopia  the  positive  part  of  accommodation — viz., 
that  employed  for  near  objects — ceases  at  an  earlier  age. 
In  emmetropia  p  is  seen  to  cross  the  zero  line  between 
the  ages  of  60  and  65  (Fig.  71)  (some  accommodation  is 
still  left,  but  it  is  employed  in  correcting  the  "  acquired 
hyperopia "),  and  in  hyperopia  even  earlier.  The 
greater  the  degree  of  hyperopia,  the  earlier  will  p  cross 


10 


146 


THE   REFRACTION   OF  THE   EYE 


the  zero  line.  In  the  case  of  a  hyperope  of  4  d  (Fig.  72) 
this  happens  between  the  ages  of  40  and  45 — that  is, 
a  hyperope  of  4  D,  when  he  reaches  the  age  of  42, 
although  he  has  some  amplitude  of  accommodation, 
has  to  make  use  of  it  entirely  for  distance;  on  the 
other  hand,  all  myopes  of  more  than  3  D  can  make 
use  of  all  their  accommodative  power  for  near  work 
(Fig.  73). 

These  conclusions  were  arrived  at  by  Bonders  from  a 
number  of  observations  made  and  recorded,  and  the 
diagrams  are  the  results  of  the  averages  of  these  observa- 
tions; but  we  now  know  that  he  under-estimated  the 
accommodative  power  between  the  ages  of  10  and  45, 
the  average  power  being  about  i'5  more.  This  mistake 
was  probably  due  to  a  majority  of  his  patients  having 
some  latent  hyperopia  which  he  was  unaware  of,  as  he 
did  not  make  his  patient  "  emmetropic  "  before  testing 
the  accommodative  power. 

Duane,  in  1909,  examined  the  accommodative  power 
of  600  patients  whom  he  had  previously  made  emme- 
tropic {Jour,  of  Amer.  Med.,  vol.  lii.,  page  1992),  and  he 
gives  the  following  tables : 


Age. 
10 

15 

20 

25 
30 
35 
40 

45 
50 
55 
60 


Minimum. 

Mean. 

Maximum 

II-2 

14 

17-5 

IO-5 

13-4 

i6-5 

9-1 

II-5 

14-2 

8-2 

IO-3 

12-9 

6-8 

8-5 

iO'6 

5-6 

7 

8-8 

4-8 

6 

7-5 

3 

3-8 

4'7 

1-4 

1-8 

2.3 

0.9 

1-3 

1-6 

0.9 

I'2 

1-5 

Following  on  the  same  lines,  I  have  examined  the 
accommodative  power  of  over  2,000  patients  with  their 
refractive  error  fully  corrected,  and  the  chart  (Fig.  74) 
shows  the  average  curve  of  my  results. 


PRESBYOPIA 


147 


/<?  /s  2  a  2s  ^(p3s  «%?  -PS  S(?  ss  eo  €6  /-a 


I 

/2 

\ 

// 

\ 

^0 

A 

»v 

0 

\ 

. 

(<? 

\ 

7 

6 

> 

s. 

\ 

V 

N 

V 

4 

3 
Z 

0 
-/ 

\ 

s. 

N 

s, 

N 

V 

N 

X 

Fig.  74. 


My  table  is  as  follows : 


Age. 

7-10 

10-15 

20 

25 

30 

35 
40 

45 
50 
55 
60 

65 


Minimum. 
9 

7 
6 

5-5 

4-5 

4 

2-5 

2 

I 

0-75 
0-50 
0*50 


Mean. 

Maximum. 

13-5 

18 

12 

18 

IO-5 

14 

9 

13-5 

7-5 

12 

6-5 

10 

5-5 

8.5 

4-25 

7 

3-5 

6 

2-5 

5 

1-75 

4 

I 

3 

The  loss  of  elasticity  of  the  lens  is  accompanied  by 
greater  firmness  and  increase  in  size,  and,  in  later  years. 


148  THE   REFRACTION   OF  THE   EYE 

by  a  loss  of  homogeneousness  and  transparency  both 
of  the  lens  and  vitreous,  which  is  such  a  striking  con- 
dition in  youth.  The  lens  reflects  more  light,  and,  by 
oblique  illumination,  often  gives  a  false  idea  of  cataract. 
At  the  age  of  45  fevery  individual  has,  more  or  less, 
about  4  D  range  of  accommodation.  From  our  formula 
a  =  p  -  r  we  get  p  =  a  +  r — i.e.,  p  =  4  +  r.  When 
the  person  is  an  emmetrope  we  can  ignore  r;  therefore 
p  =  4.  Now  P,  the  punctum  proximum,  is  '-^^; 
therefore  P  =  ub^  =  25  cms.  That  is,  the  average 
emmetrope  at  the  age  of  45  cannot  read  nearer  than 
25  cms.,  or  10  inches.  This  may  be  taken  as  the 
presbyopic  point. 

Donders  fixed  the  presbyopic  point  at  22  cms.,  the 
point  to  which,  he  said,  the  average  individual's  near 
point  had  receded  at  the  age  of  40. 

As  a  matter  of  fact,  there  is  no  fixed  presbyopic  point. 

An  individual  may  be  said  to  have  arrived  at  the 
presbyopic  period  when  he  finds  that  he  cannot  do  his 
near  work  for  any  length  of  time  without  discomfort 
or  some  symptom  of  strain.  A  long- armed  man  who 
does  very  little  near  work  may  not  require  glasses  until 
the  age  of  50,  whereas  a  seamstress,  with  the  same  re- 
fraction and  of  the  same  age,  may  have  had  to  take  to 
glasses  five  years  earlier. 

Some  writers  object  to  the  term  "  presbyopia,"  and 
would  expunge  it  from  ophthalmology,  giving  us  nothing 
in  its  place.  We  must  have  some  term  to  express  that 
condition  in  which,  as  the  result  of  the  increase  of  years, 
the  range  of  accommodation  is  diminished  and  the  vision 
of  near  objects  is  interfered  with. 

The  recognition  of  presbyopia  is  not  difficult.  When 
a  patient  complains  that  he  has  to  hold  his  book,  when 
reading,  further  away  than  he  has  been  accustomed  to, 
that  this  is  more  especially  so  by  artificial  light,  that  the 
figures  3,  5,  and  8  become  confused,  and  that  n  and  u  are 
difficult  to  distinguish,  and  at  the  same  time  asserts 


PRESBYOPIA  149 

that  his  distant  vision  has  not  altered,  we  may  be  almost 
certain  we  are  dealing  with  a  presbyope. 

When  a  patient  whose  near  point  has  receded,  say,  to 
33  cms.  attempts  to  read  or  work  at  that  distance  for 
any  length  of  time,  symptoms  of  eyestrain  will  be  sure 
to  supervene.  It  is  a  fact  that  we  get  from  everyday 
experience,  that  the  full  power  of  a  muscle  can  be  exer- 
cised only  for  a  very  short  time  without  fatigue.  A 
person  whose  near  point  is  at  33  cms.  is  using  the  whole 
power  of  his  ciliary  muscle  in  order  to  focus  an  object 
at  that  distance  on  his  retina,  and  fatigue  of  the  muscle 
will  very  soon  ensue.  This  fatigue  causes  the  muscle  to 
relax,  it  cannot  contract  to  its  full  extent;  vision  then 
becomes  hazy,  and  becomes  distinct  again  only  when 
the  object  has  been  further  removed  from  the  eye.  At 
the  same  time  the  patient  will  probably  complain  that, 
after  reading  some  little  time,  headache  comes  on,  and 
the  eyes  begin  to  water;  these  temporary  symptoms  of 
eyestrain  will  pass  into  chronic  symptoms  in  time,  and 
the  red,  irritable-looking,  watery  eyes  of  middle-aged 
people  are  often  due  to  this  cause. 

Treatment. — Two  classes  of  patients  come  for  treat- 
ment. One  class  simply  requests  glasses  for  reading 
(some  of  these  have  been  to  an  optician  and  have 
themselves  selected  a  weak  spherical  glass  which  proves 
unsuitable) ;  the  other  class  comes  complaining  of  some 
symptom  of  eyestrain,  and  they  may  or  may  not  be 
aware  that  they  require  glasses. 

A  cycloplegic  is  rarely  necessary  (except  in  isolated 
cases  where  our  results  are  unsatisfactory  or  we  wish 
to  examine  the  lens  for  cataract),  because  ciliary  spasm 
is  very  unlikely  to  be  present,  and  there  is  no  latent 
hyperopia,  it  having  all  become  manifest.  We  first 
ascertain  the  distant  correction,  and  then  the  near 
point  of  distinct  vision  and  the  most  comfortable 
distance  for  reading  or  working.  Suppose  distant  vision 
is  normal  and  the  punctum  proximum  is  28  cms.,  and 


150  THE  REFRACTION   OF  THE   EYE 

the  distance  at  which  the  patient  wishes  to  read  is 
33  cms.,  his  ampUtude  of  accommodation  is  -^  =  3*5  d  ; 
to  avoid  fatigue  he  must  not  use  the  whole  of  this,  but 
must  keep  about  J  in  reserve.  Let  him  have  i*5  .d  in 
reserve;  this  leaves  him  2  D  available  accommodative 
power,  and,  as  he  requires  3  d,  we  supply  the  deficit  by- 
giving  him  +1  D  glasses. 

The  treatment,  simple  as  it  appears,  is  not  always 
successful,  because  the  accommodative  power  varies 
with  the  individual,  and  when  it  is  low  a  greater  reserve 
has  to  be  left.  We  generally  find  that  an  emmetrope 
of  about  48  requires  a  reading  glass  of  +1  D,  and  an 
additional  +1  for  every  five  years. 

Perusal  of  the  table  on  page  147  shows  what  a  wide  difference 
there  is  in  the  accommodative  power  of  different  individuals; 
when  the  accommodative  power  is  lower  than  normal  it  will  be 
found  that  the  individual  is  older,  and  looks  older,  than  his  age, 
and  vice  versa.  Premature  presbyopia,  which  is  really  pre- 
mature senility,  is  provoked  by  various  conditions,  but  intestinal 
stasis  is  perhaps  the  commonest  cause.  A  small  error  of  refrac- 
tion uncorrected  not  only  tends  to  lower  the  power  of  the  ciliary 
muscle  by  the  constant  drain  on  its  energy,  but  also  tends  to 
hasten  the  sclerosing  processes  in  the  lens  (see  page  163). 

When  ametropia  is  present,  the  distance  correction 
must  be  ascertained,  and  the  presbyopic  glasses  added 
to  it;  thus,  if  the  distance  glass  is  +2,  and  an  addition 
of  +1*5  is  required  for  reading,  the  reading  glass  is 
+3-5;  if  the  distance  correction  is  -  4,  and  an  addition 
of  -I-  2  is  required  for  reading,  the  reading  glass  is  -  2 . 
When  astigmatism  is  present,  the  correcting  cylinder 
must,  of  course,  be  added. 

It  was  the  custom  until  quite  recently  for  presbyopes 
who  had  fairly  normal  distant  vision  to  be  px^ovided  with 
reading  glasses  only,  but  we  now  recognize  the  impor- 
tance of  correcting  all  errors  of  refraction,  especially  at 
this  period  of  life,  so  that  unless  the  patient  is  an  emme- 
trope he  will  require  glasses  for  distance  as  well  as  near 
work.    This  double  correction  is  best  prescribed  in  the 


PRESBYOPIA  151 

form  of  "  bi-focal  '*  spectacles  or  pince-nez  (see  page  17), 
where  the  upper  part  is  used  for  distance  and  the  lower 
for  reading.  These  bi-focals  are  now  made  so  that  the 
division  between  the  two  portions  is  invisible,*  and  it  is 
not  an  exaggeration  to  assert  that  they  have  completely 
revolutionized  the  treatment  of  presbyopia. 

They  should  be  prescribed  for  all  patients  who  have 
the  slightest  error  of  refraction  in  addition  to  their 
presbyopia,  if  they  show  any  sign  of  eyestrain  or  nerve 
waste.  On  the  other  hand,  those  who  live  an  open-air 
life,  whose  distant  vision  is  practically  normal,  and  who 
^i^joy  good  health,  may  be  allowed  to  have  reading 
glasses  only. 

As  the  bi-focals  have  to  be  used  for  writing  as  well  as 
reading,  it  is  very  important  that  the  lower  section  be 
not  too  strong.  It  is  rarely  necessary  at  any  time  of 
life  to  wear  a  stronger  addition  than  3  d  ;  should  the 
patient  want  a  stronger  reading  glass  for  the  evening,  it 
must  be  prescribed  as  a  separate  glass.  As  an  alterna- 
tive to  bi-focals,  the  reading  "  addition "  may  be 
mounted  in  pince-nez  or  "  hook  fronts,"  to  be  worn  in 
front  of  the  distance  glasses  when  reading.  When 
so-called  "  music  "  glasses  for  near  work  at  arm's  length 
are  required,  they  should  be  +1  or  -hi'5  D  weaker  than 
the  reading  glasses. 

Fatigue  of  the  Accommodation. — We  must  be  careful 
to  distinguish  between  presbyopia  and  ordinary  physio- 
logical fatigue  of  the  ciliary  muscle,  which  may  come 
on  after  prolonged  use  of  the  eyes  in  the  normal,  or 
may  be  associated  with  general  muscle  fatigue,  as  in 
neurasthenia.  As  we  have  seen,  presbyopia  is  not 
necessarily  due  to  weakness  of  the  ciliary  muscle,  but 
to  the  sclerosing  of  the  lens. 

*  The  optician  must  take  great  care  in  fitting  and  centring 
bi-focals ;  the  division  between  the  glasses  should  never  be  higher 
than  the  margin  of  the  lower  lid,  and  full  allowance  should  always 
be  made  for  convergence — thus,  if  the  patient  converges  3  mms., 
then  each  reading  portion  should  be  decentred  in  i'5  mms. 


CHAPTER  X 

ANISOMETROPIA 

Anisometropia  is  a  condition  in  which  the  refraction 
of  the  two  eyes  is  different.  A  difference  in  refraction 
in  the  two  eyes  is  more  often  met  with  than  absolute 
equaUty,  and  in  astigmatism  it  is  very  common  to  find 
a  difference  of  *2^  or  •50  between  them. 

It  was  formerly  the  practice  only  to  recognize  aniso- 
metropia when,  to  produce  the  maximum  of  visual 
acuity  in  the  two  eyes,  different  glasses  were  required; 
but  the  increased  knowledge  we  have  now  of  the  effects 
of  eyestrain  has  taught  us  that  the  smallest  amount 
must  not  be  neglected,  and,  further,  that  the  smaller 
the  amount  of  anisometropia,  the  more  important  is  it 
to  correct  it. 

Every  possible  combination  may  exist: 

1.  One  eye  may  be  emmetropic,  and  the  other  ame- 
tropic. 

2.  Both  eyes  may  be  ametropic — 

{a)  The  same  variety  of  ametropia,  but  unequal 
in  degree; 

{b)  Different  varieties  of  ametropia,  one  eye 
myopic  and  the  other  hyperopic ;  this  variety 
is  sometimes  called  "  antimetropia." 

When  one  eye  is  astigmatic  and  the  other  hyperopic 
or  myopic,  the  astigmatic  eye  has  generally  the  same 
form  of  ametropia  as  the  non-astigmatic  eye,  and  very 
often  one  of  its  meridians  has  the  same  amount  of 
ametropia. 

152 


ANISOMETROPIA  153 

Except  when  it  is  the  consequence  of  an  operation, 
loss  of  lens,  etc.,  anisometropia  may  be  regarded  as  con- 
genital, and  attributable  to  the  unequal  development  of 
the  eyes. 

The  difference  of  the  refraction  of  the  two  eyes  may  be 
very  great,  and  is  then  often  associated  with  marked 
asymmetry  of  the  face;  a  difference  of  lo  dioptres  has 
been  recorded. 

Varieties  of  Anisometropia. — There  are  three  varieties 
of  anisometropia: 

1.  Simultaneous  binocular  vision  exists. 

2.  The  eyes  are  used  alternately. 

3.  One  of  the  eyes  is  permanently  excluded. 

I.  Simultaneous  Binocular  Vision  —  Tests  for 
Binocular  Vision — {a)  The  Prism  Test. — The  patient 
fixes  an  object  at  a  distance,  and  a  strong  prism,  base 
in  or  out,  is  placed  before  one  eye,  the  other  being 
uncovered;  at  the  moment  of  interposing  the  prism,  if 
the  eye  make  a  movement  towards  the  apex  of  the 
prism,  binocular  vision  exists. 

(h)  Snellen's  Coloured  Glass  Test* — "  Friend  "  Test. — 
This  apparatus  consists  of  a  frame,  hung  up  before  the 
window,  in  which  letters  of  coloured  glass,  alternately 
green  and  red,  are  placed.  The  patient  standing  in 
front  of  them,  at  a  distance  of  4  or  5  metres,  is  provided 
with  a  spectacle  frame  into  which  one  red  and  one 
green  glass  is  placed,  the  colour  of  these  glasses  being 
of  the  same  intensity  as  that  of  the  letters.  Only 
the  red  letters  can  be  seen  through  the  red  glass,  and 
the  green  letters  through  the  green  glass,  so  that  each 
eye,  separately,  only  sees  half  the  letters.  The  letters 
in  the  frame  may  be  made  to  spell  a  word,  such  as 
FRIEND,  so  arranged  that  the  letters  f,  i,  and  N  are 
red,  and  R,  e,  and  d  green.     If  the  patient,  having  had 

*  This  test  was  introduced  into  England  from  Bonders' 
clinique  in  1881  by  the  author. 


154  THE   REFRACTION   OF  THE   EYE 

any  ametropia  corrected,  sees  all  the  letters  and  spells 
the  word  "friend,"  we  know  he  has  binocular  vision; 
but  if,  e.g.,  he  only  spells  f  i  n,  we  know  that  he  is  using 
the  eye  with  the  red  glass  in  front  of  it,  and  that  the 
other  eye  is  excluded  from  vision. 

For  binocular  vision  to  exist  in  anisometropia  the 
difference  in  refraction  between  the  two  eyes — that  is, 
the  degree  of  anisometropia — must  be  small,  although 
cases  have  been  recorded  where  it  has  amounted  to  6  d. 
Under  these  circumstances,  although  the  magnitude  and 
acuteness  of  the  images  in  the  two  eyes  are  unequal, 
they  overlap  and  help  each  other. 

If  each  ciliary  muscle  could  act  independently,  the 
anisometrope  could  very  often  correct  each  eye  by  a 
separate  and  independent  accommodation  in  each  eye; 
but  it  is  generally  believed  that  the  same  effort  of 
accommodation  is  made  on  both  sides,  with  the  result 
that  only  one  image  is  sharp.  At  the  same  time,  it 
should  be  noted  that  when  the  anisometropia  is  of  low 
degree,  some  patients  may  have  the  power  of  producing 
asymmetrical  accommodation  to  a  limited  extent.  If  we 
place  a  convex  glass  of  +»5o  in  front  of  one  eye  and  a 
concave  glass  of  -  *$o  in  front  of  the  other,  and  thus 
produce  an  anisometropia  of  i  d,  and  attempt  to  read 
with  the  glasses,  a  very  distinct  feeling  of  strain  is  ex- 
perienced; but  whether  this  strain  is  brought  about  by 
actual  asymmetrical  accommodation,  or  only  by  the 
attempt  to  produce  it,  it  is  difficult  to  say. 

Treatment. — The  treatment  in  these  cases  varies  con- 
siderably. In  quite  young  patients,  unless  the  difference 
between  the  eyes  is  very  great,  the  full  correction  of  each 
eye  should  be  ordered  in  accordance  with  the  plans  stated 
in  the  previous  pages.  In  older  patients  we  must  try 
the  binocular  effect  before  prescribing  glasses.  The  full 
correction  in  each  eye  is  especially  indicated  when 
marked  symptoms  of  eyestrain  exist,  and  in  these  cases, 
although  the  treatment  may  be  complained  of  at  first. 


ANISOMETROPIA  155 

it  is  wise  to  insist  upon  it,  and  in  many  cases  the  dis- 
comfort produced  at  first  by  the  glasses  disappears  in  a 
few  weeks,  and  eventually  the  symptoms  of  eyestrain 
disappear.  On  the  other  hand,  patients  who  have  lived 
many  years  without  having  their  anisometropia  cor- 
rected, have  become  so  accustomed  to  the  difference 
between  the  eyes  that  the  removal  of  this  difference  not 
only  confers  no  benefit,  but  proves  irksome,  and  they 
may  complain  of  dazzling,  giddiness,  and  headache. 

Place  the  proper  correction  in  front  of  each  eye,  and 
try  the  binocular  effect;  if  this  is  unsatisfactory,  take 
off  a  little  from  the  stronger  glass  or  add  a  little  to  the 
weaker,  or  do  both.  Sometimes  we  find  that  the  patient 
prefers  the  same  correction  in  both  eyes,  and  that  this 
correction  is  weaker  than  that  required  by  the  more 
ametropic  eye.  For  instance,  suppose  the  right  eye 
is  emmetropic  and  the  left  hyperopic  to  the  extent  of 
2  D,  we  try  first  a  plane  glass  in  front  of  the  right  and 
+2  in  front  of  the  left;  if  this  causes  discomfort,  we 
may  try  a  +»5  in  front  of  the  right  and  +i«5  in  front 
of  the  left.  If  this  is  still  uncomfortable,  we  try  4-1  in 
front  of  both  eyes.  No  definite  rule  can  be  laid  down; 
each  case  should  be  treated  according  to  its  require- 
ments and  the  patient's  sensations. 

It  is  a  good  plan  when  the  correction  has  been  found 
under  a  cycloplegic  to  deduct  a  little  more  from  the 
more  ametropic  eye  and  a  little  less  from  the  other. 
For  instance,  supposing  under  atropine  the  refraction 
is,  right  eye  +2,  and  left  eye  -i-3'5;  we  give  +i«25 
for  the  right,  and  +2  for  the  left.  When  the  difference 
between  the  glasses  is  great,  the  patient  should  be 
taught  to  turn  his  head  when  looking  to  the  right  or  left, 
and  not  his  eyes,  so  that  he  is  always  looking  through 
the  centre  of  his  glasses.  If  he  does  not  look  through 
the  centre  of  his  glasses,  a  prismatic  effect  is  produced; 
this,  of  course,  will  produce  an  artificial  heterophofia 
and  cause  strain.     Mr.  W.  A.  Dixey  has  suggested  that 


156 


THE   REFRACTION   OF  THE   EYE 


this  prismatic  effect  on  looking  eccentrically  can  be 
avoided  by  reducing  the  margin  of  the  stronger  lens  to 
the  power  of  the  weaker  one  in  myopic  patients,  where 
the  difference  in  the  two  eyes  is  marked  and  binocular 
v'sion  exists.  For  instance,  the  right  eye  is  -2.  and 
the  left  -  6 ;  the  outer  margin  of  the  -  6  lens  is  reduced 
to  -2,  as  in  Fig.  75. 

2.  The  Eyes  are  used  alternately. — One  eye  is 
emmetropic  or  slightly  hyperopic,  and  is  used  for 
distance,  and  the  other  myopic,  and  used  for  near  work. 
If  eyestrain  be  not  present,  the  patient  may  prefer  to 
have  no  glasses.     Although  he  has  lost  binocular  vision, 


Cz 


=a 


Fig.  75. 


he  has  gained  other  advantages;  in  some  cases  he  can 
entirely  dispense  with  muscular  effort,  his  ciliary  muscle 
and  internal  recti  being  rarely  used.  If  eyestrain  be 
present,  we  must  prescribe  glasses  after  ascertaining  the 
binocular  combination  that  suits  best.  We  sometimes 
find  that  if,  for  instance,  both  eyes  are  hyperopic,  the 
patient  prefers  the  same  glass  in  both  eyes  corresponding 
to  that  required  by  the  most  ametropic;  this  latter  eye 
will  then  be  used  for  distance  and  the  other  for  near 
work.  In  the  same  manner,  if  both  eyes  are  myopic, 
we  give  each  the  correcting  glass  of  the  weaker. 

In  all  these  cases  the  cylinder  should  never  be  altered. 


ANISOMETROPIA  157 

except  in  rare  instances  where  there  is  a  great  difference 
in  the  astigmatism  of  the  two  eyes,  and  then  the  stronger 
cyhnder  may  have  to  be  reduced. 

3.  One  Eye  is  permanently  excluded  from 
Vision. — ^When  the  difference  between  the  eyes  is 
great,  the  more  defective  eye  is  Httle  used,  and  tends 
to  become  amblyopic,  if  it  is  not  so  already.  In  such 
cases  we  must  give  each  eye  its  proper  correction,  and 
instruct  the  patient  to  practise  the  amblyopic  eye  by 
totally  excluding  vision  with  the  good  eye  by  covering 
it  with  a  patch  for  a  certain  time  every  day.  In  "  am- 
blyopia ex  anopsia,"  occurring  in  young  patients  with 
convergent  strabismus,  this  treatment,  patiently  perse- 
vered in,  is  often  most  satisfactory. 

The  defective  eye  may  never  take  its  proper  place  in 
binocular  vision,  but  in  some  cases  it  may  become  very 
useful,  especially  if  any  damage  or  disease  should  affect 
the  good  eye;  and,  moreover,  the  cosmetic  effect  which 
sometimes  occurs  is  considerable,  for,  if  treated  in  time, 
the  strabismus  which  so  often  appears  in  these  cases 
may  be  prevented. 

Adults  with  only  one  useful  eye  may  be  allowed  to 
wear  a  monocle,  provided  that  the  correction  does  not 
include  a  low-power  cylinder.  If  a  cylinder  is  required, 
the  glass  must  always  be  placed  in  the  proper  position : 
and  if  a  high  cylinder  is  used,  the  patient  can  tell  at  once 
if  the  axis  of  the  cylinder  has  been  properly  adjusted. 
But  in  a  cylinder  of  '25  or  '5  it  will  be  impossible  for 
him  thus  to  judge  the  correctness  of  the  axis,  even 
though  he  thinks  he  is  putting  the  glass  into  its  proper 
position.  In  such  cases,  if  a  monocle  is  insisted  upon, 
the  cylinder  should  be  omitted. 

Monocles  should  be  mounted  in  a  frame  with  a  bracket, 
in  order  that  the  glass  may  be  removed  from  contact  with 
the  skin  and  lashes. 

Presbyopia  and  Anisometropia. — When  presbyopia  is 
present  with  anisometropia,  we  should,  whenever  pes- 


158  THE   REFRACTION   OF   THE   EYE 

sible,  prescribe  bi- focal  glasses.  The  lower  reading 
section  must  be  found  by  the  same  kind  of  trial  as  we 
made  for  the  distant  correction.  It  does  not  follow  that 
the  same  addition  is  given  to  each  eye.  For  instance, 
one  eye  has  a  hyperopia  of  2  and  the  other  of  4,  and  the 
patient  is  52;  we  should  try  +4  and  +6  for  reading, 
or,  if  this  be  not  comfortable,  +4»5  and  +5 '5,  or  +5 
in  both  eyes. 

When  one  eye  is  permanently  excluded,  and  the  re- 
maining eye  is  ametropic  and  presbyopic,  as  in  aphakia 
after  cataract  extraction,  reversible  spectacles  are  useful ; 
the  distant  correction  is  on  one  side  and  the  reading 
on  the  other.  When  the  patient  is  walking,  the  distant 
correction  is  in  front  of  the  "  good  "  eye  and  the  reading 
glass  in  front  of  the  useless  eye,  and  when  he  wishes  to 
read  he  reverses  the  spectacles,  and  so  brings  the  read- 
ing-glass in  front  of  the  good  eye,  the  bridge  being 
made  to  fit  the  nose  in  either  position. 

When  the  difference  between  the  two  eyes  is  marked,  and 
simultaneous  binocular  vision  exists,  although  the  distance  cor- 
rection is  accepted,  discomfort  may  come  with  near  work.  This 
is  due  to  the  production  of  an  artificial  hyperphoria.  Let  us 
suppose  the  right  eye  to  have  a  myopia  of  i'5,  and  the  left  a 
hyperopia  of  the  same  amount.  When  reading  and  looking,  say, 
6  mm.  below  the  optical  centres,  the  patient  is  looking  through 
a  prism  1°  base  down  in  front  of  the  right  eye  and  1°  base  up 
in  front  of  the  left.  (A  lens  i  d,  decentred  8*7  mm.,  produces  a 
prismatic  effect  of  i°.)  The  difficulty  can  be  overcome  by 
cementing  on  the  lower  portion  of  each  lens  the  necessary  correct- 
ing prism.  In  the  above  case  a  prism  1°  base  up  in  front  of  the 
right  eye  and  1°  base  down  in  front  of  the  left  will  correct  the 
hyperphoria. 

When  a  presbyopic  correction  is  also  required,  the  prismatic 
effect  may  be  obtained  by  decentring,  or  by  making  the  prism 
convex  to  the  amount  required. 


CHAPTER  XI 

APHAKIA 

Although,  to  be  quite  correct,  aphakia  denotes  an 
eye  without  a  lens,  the  latter  having  been  purposely 
removed  by  operation,  or  accidentally  lost  through  a 
perforating  wound  or  ulcer,  we  generally  include  under 
aphakia,  conditions  in  which  the  lens  is  more  or  less  com- 
pletely dislocated  (as  in  the  old  operation  of  "  couch- 
ing "),  so  that  rays  of  light  passing  into  the  eye  do  not 
intercept  any  portion  of  it. 

The  absence  of  the  lenticular  images,  found  by  holding 
a  light  near  to  the  eye  (see  page  31),  and  a  tremulous 
iris,  are  characteristic  signs  of  this  condition. 

A  strong  convex  glass  must  be  placed  before  the  eye 
to  obtain  distinct  distant  vision,  unless  the  original 
condition  of  the  eye  was  one  of  high  myopia.  A  convex 
glass  of  II  or  12  d,  placed  about  13  mm.  in  front  of  the 
eye,  is  about  the  equivalent  of  the  crystalline  lens ;  but, 
"  as  the  refracting  system  of  the  eye  is  now  entirely 
different  from  that  of  an  ametropic  eye,  the  tests  of 
visual  acuteness  are  no  longer  comparable  with  that  of 
normal  eyes.  The  retinal  image  in  an  aphakic  eye  is 
I •33  times  larger  than  in  a  normal  eye.  Hence  vision 
of  f  in  a  corrected  aphakic  would  really  only  correspond 
to  a  visual  acuteness  of  A  in  a  normal  eye  "  (Percival). 

As  all  accommodation  is  lost  through  the  removal  of 
the  lens,  the  convex  glass  has  to  be  increased  in  strength 
for  near  work,  according  to  the  distance  of  the  near 
work  from  the  eye ;  thus,  if  33  cms.  is  the  spot  required 

159 


l60  THE   REFRACTION    OF  THE   EYE 

at  which  to  read,  +  3  must  be  added;  and  if  25  cms.  is 
the  distance,  +  4  must  be  added.  After  cataract  ex- 
traction there  is  almost  always  a  large  amount  of  inverse 
astigmatism,  and  a  +  cylinder  of  i  to  2  d,  placed 
horizontally,  or  nearly  horizontally,  will  probably  be 
required.  As  this  inverse  astigmatism  is  generally 
fairly  high  immediately  after  the  operation,  and  gradu- 
ally diminishes  during  a  period  of  two  or  three  months, 
it  is  wise  not  to  prescribe  the  cataract  glasses  until 
this  period  has  elapsed. 

When  a  cylinder  is  required,  the  glass  is  best  given 
in  the  sphero-toric  form  (see  page  16). 

When  the  original  condition  of  the  eye  was  one  of 
ametropia,  the  following  formula  is  a  fairly  accurate 
guide  of  the  glasses  required  after  operation  (Percival) : 

-,25 +D 


1(T0 


2 


where  D  is  the  original  refraction  of  the  eye  and  D' 
is  the  approximate  glasses  required;  thus,  if  D  =  -  15, 
then 

TA/         25 +  (-15)     25-15      10  , 

2-i^f  2+-I5     2-15  ^    ^' 

and  if  the  original  refraction  was  a  hyperopia  of  6, 

^,         25+6         31         31 
2-Tw    2 --06    1-94 

This  formula  is  calculated  for  the  position  of  the  cor- 
recting lens  D  being  13*7  mm.  in  front  of  the  eye;  if 
this  distance  is  increased,  the  convex  glass  is  propor- 
tionately decreased  in  strength.  For  practical  purposes, 
if  we  halve  the  amount  of  the  original  error  and  add 
to  it  +  II,  we  shall  find  that  this  is  approximately  the 
glass  required.    According  to  this,  if  the  patient  had 

*  This  sign  represents  approximate  equivalency. 


APHAKIA  l6l 

originally  a  myopia  of  22,  he  probably  requires  no  dis- 
tance glass  after  the  removal  of  the  lens. 

The  most  convenient  form  of  glass  is  the  bi-focal  (see 
page  16) .  Some  patients  prefer  the  reversible  form  (see 
page  158). 

If  the  correction  be  not  satisfactory,  an  examination 
of  the  patient  in  the  dark  room  with  focal  illumination 
and  a  strong  lens,  will  probably  reveal  an  opaque  cap- 
sule, which  will  require  needling  before  any  good  result 
is  obtained  with  glasses. 


IX 


CHAPTER  XII 

EYESTRAIN 

Eyestrain,  as  we  now  understand  it,  extends  over  a 
far  wider  field  than  it  did  even  twenty-five  years  ago, 
and  the  list  of  troubles  which  more  or  less  depend  upon 
its  presence  grows  larger  every  year.  It  includes,  of 
course,  the  old  "  asthenopia "  that  Bonders,  writing 
in  1858,  considered  was  due  to  hyperopia,  and  the 
"  muscular  asthenopia  "  which  Von  Graefe,  a  few  years 
later,  attributed  to  strain  of  the  internal  recti  muscles. 
The  word  "  asthenopia,"  by  its  derivation,  denotes 
weak  sight,  which,  in  the  large  majority  of  cases,  does 
not  exist;  and  even  if  we  take  the  broader  meaning, 
implying  tired  sight,  the  word  is  equally  inappropriate. 
It  is  therefore  best  to  restrict  the  term  "  asthenopia  " 
entirely  to  retinal  fatigue — i.e.,  retinal  asthenopia — 
with  which  we  are  not  here  concerned. 

Eyestrain  may  be  defined  as  a  symptom  or  group  of 
symptoms  produced  by  the  correction  or  attempt  at 
correction  by  the  ciliary  muscle  of  an  error  of  refraction 
or  a  small  amount  of  anisometropia,  or  as  a  want  of 
balance  between  the  external  muscles  of  the  eye.  Where 
gross  errors  exist,  either  in  the  refraction  or  the  aniso- 
metropia or  in  the  muscular  equilibrium,  the  patient 
cannot  correct,  and  consequently  makes  no  attempt 
to  correct,  the  defect,  and  eyestrain  is  not  produced. 
The  smaller  the  error,  the  more  likely  is  eyestrain  to  be 
present,  and  also,  unfortunately  for  the  patient,  the 
more  likely  is  it  to  be  overlooked. 

162 


EYESTRAIN  163 

The  symptoms  of  eyestrain  may  be  grouped  under 
three  headings : 

1.  Manifestations  on  the  eye  and  Uds. 

2.  Peripheral  irritation. 

3.  Nerve  exhaustion. 

1.  Manifestation  of  Eyestrain  on  the  Eye  and  Lids. — 

Eyestrain  means  an  increased  demand  for  work  on  the 
part  of  the  cihary  or  external  muscles,  which  determines 
an  increased  flow  of  blood  to  these  parts;  if  this  is 
constant,  congestion  is  liable  to  follow,  and  with  it  pain. 
With  the  parts  thus  rendered  specially  receptive  to 
infective  germs,  there  is  little  difficulty  in  understanding 
that  inflammation  should  ensue,  and  thus  we  find  that 
blepharitis,  conjunctivitis,  corneal  ulcers,  phlyctenulae, 
iritis,  cyclitis  and  glaucoma  may  have  eyestrain  as  one 
of  the  predisposing  causes.  There  is  also  little  doubt 
that  cataract  may  be  started  by  the  irregular  contrac- 
tion of  the  ciliary  muscle  in  correcting  low  errors  of 
astigmatism,  and  that  the  correction  of  these  errors, 
with  the  consequent  disappearance  of  the  eyestrain,  will 
stay  the  progress  of  this  disease. 

2.  Peripheral  Irritation — 

(a)  With  pain. 

(b)  Without  pain. 

Peripheral  Irritation  with  Pain. — In  the  same  manner 
that  a  decayed  molar  may  produce  neuralgia  by  peri- 
pheral irritation,  so  eyestrain  may  produce  headaches, 
or  any  other  form  of  peripheral  irritation.  Headache 
is  by  far  the  commonest  symptom  of  eyestrain.  So 
common  is  it,  indeed,  that  no  physician  should  attempt 
to  treat  a  patient  for  constantly  recurring  headaches 
unless  the  existence  of  eyestrain  has  been  eliminated  by 
proper  correction  under  a  cycloplegic. 

The  position  and  character  of  the  headache  form  no 
guide  to  the  cause,  for  ocular  headache  may  be  of  any 
possible  variety.     It  may  simply  amount  to  a  slight 


164  THfi  REFRACTION   OF  THE  EYE 

aching  over  the  eyes  or  at  the  back  of  the  orbit.  It  may 
be  a  frontal  headache.  Sometimes  it  originates  and 
remains  Hmited  as  a  vertical  or  occipital  pain,  or  it 
may  originate  in  the  brow,  and  pass  to  the  vertex  and 
occiput,  and  pass  down  the  spine.  It  may  be  unilateral 
as  a  typical  hemicrania,  and  may  be  indistinguishable 
from  a  true  migraine  attack. 

There  is  no  rule  as  to  its  position;  this  varies  with 
the  individual.  In  some  it  is  superficial,  akin  to  neu- 
ralgia; in  others,  deep-seated. 

It  may  be  a  dull,  heavy  ache,  difficult  to  localize 
accurately,  or  it  may  be  a  sharp,  shooting,  lancinating 
pain,  that  seems  to  originate  in  a  tender  spot  on  the 
scalp  or  forehead.  Some  describe  the  pain  as  an  opening 
and  shutting  of  the  skull,  others  as  if  a  nail  were  being 
driven  into  the  vertex. 

But  the  commonest  form  of  eyestrain  headache  is  a 
pain  over  one  or  both  brows,  often  termed  "  brow  ague." 

Again,  the  time  of  the  headache  varies;  it  may  be  a 
permanent  headache  or  periodic,  and  it  may  appear  to 
have  no  direct  association  with  excessive  use  of  the 
eyes. 

Early-morning  headache  is  a  very  common  form  of 
ocular  headache,  and  this  astonishes  most  patients,  as 
they  imagine  that  the  night's  rest  should  have  removed 
the  possibility  of  this. 

To  explain  the  periodicity  of  the  headaches  we  have 
to  remember  that  the  cause  of  the  headache  may  be 
multiple.  There  may  be  two,  or  even  three,  factors 
present.  A  patient  who  suffers  from  a  periodical  head- 
ache may  have  eyestrain,-  a  gouty  or  other  diathetic 
tendency,  and  at  times,  added  to  these,  some  special 
nerve  depressant,  such  as  worry  or  trouble. 

Now,  each  of  these  factors,  or  possibly  any  two,  may 
not  suffice  to  cause  a  headache;  all  three  must  be 
present,  and  only  at  such  times  as  all  three  are  present 
is  the  headache  there.     If  the  eyestrain  be  removed  by 


EYESTRAIN  165 

correcting  the  error,  the  other  causes,  save  under  ex- 
ceptional conditions,  will  never  succeed  in  producing 
the  pain. 

It  is  important  to  remember  that  with  this  headache 
there  is  often  associated  nausea,  and  even  vomiting. 
The  so-called  biliotis  headache  of  the  "  old  school  "  is 
generally  an  ocular  headache. 

The  intimate  relation  of  the  nerve-supply  of  the  ocular 
muscles  with  the  fifth  nerve,  and  the  association  of  the 
latter  with  the  sympathetic  and  pneumogastric,  explain 
the  method  of  origin  of  the  symptoms. 

The  ciliary  muscle  and  the  external  eye  muscles, 
when  strained,  demand  an  increased  supply  of  blood, 
which  in  time  leads  to  congestion.  This  in  its  turn  will 
not  be  limited  to  the  strained  part,  but  will  spread  to 
other  parts  of  the  eye,  causing  the  watery,  red  eye 
already  alluded  to.  The  pain  of  "  fatigue  "  is  probably 
due  to  this  congestion,  and  accounts  for  the  tender 
eyeballs  so  common  in  migraine. 

Liveing  says  that  the  nerve-storms  in  migraine  have 
their  point  of  departure  or  principal  focus  in  the  optic 
thalami,  and  that  the  normal  course  is  from  above 
downwards  to  the  nuclei  of  the  vagi,  and  from  before 
backwards  in  the  sensory  tract,  thus  explaining  the 
peculiar  visual  phenomena,  such  as  teichopsia  and  other 
symptoms  of  ophthalmic  migraine. 

Of  course,  there  are  cases  of  true  migraine,  when  the 
"  point  of  departure  "  is  from  above  downwards,  but  a 
very  large  percentage  of  cases  labelled  "  migraine  "  are 
purely  ocular  in  origin;  and  if  the  error  of  refraction  is 
corrected,  or  the  muscular  equilibrium  re-established, 
the  symptoms  disappear. 

The  connection  of  the  fifth  nerve  with  the  sympathetic 
enables  us  to  understand  how  the  peripheral  irritation 
of  eyestrain  can  pass  to  the  dura  mater,  pia  mater,  and 
sensory  layer  of  the  brain  cortex. 

Gowers  says:  "  If  the  sensory  cells  of  the  cortex,  in 


l66  THE  REFRACTION   OF  THE  EYE 

which  the  cranial  and  intracranial  sensitive  structures 
are  represented,  are  the  most  readily  influenced  of  all 
the  sensory  cells,  we  can  understand  that  headache 
should  result  from  vascular  repletion."*  Everyone 
knows  from  experience  that  a  headache  increases  some- 
times to  an  alarming  severity  when  the  head  is  lowered — 
i.e.,  when  more  blood  flows  to  the  head. 

As  we  have  already  seen,  although  in  eyestrain  the 
peripheral  irritation,  if  it  takes  the  above  form,  is  always 
or  almost  always  present,  it  may  not  manifest  itself 
as  a  headache  unless  there  is  a  general  increase  of  blood- 
pressure.  If  the  blood-pressure  be  lowered  by  general 
treatment,  the  headache  will  disappear.  But  this  does 
not  mean  that  the  headache  was  not  ocular  in  origin. 
It  simply  means  that  one  of  the  factors  causing  the 
pain  has  been  removed,  and  the  other,  or  others,  are  not 
sufficient  to  cause  it. 

It  is  therefore  a  good  rule  never  to  attempt  to  treat 
a  headache  as  a  migraine  without  previously  eliminating 
eyestrain. 

Peripheral  Irritation  of  Eyestrain  unaccompanied  by 
Pain. — The  chief  types  of  this  form  of  eyestrain  are 
epileptic  attacks  and  choreiform  movements  of  the  facial 
muscles. 

It  is  important  to  remember  that  it  is  not  the  error 
of  refraction  that  causes  the  peripheral  irritation,  but 
the  unconscious  correction  of  the  error  by  the  patient. 
When  the  defect  is  great,  no  attempt  is  made  to  correct 
it,  as  the  ciliary  muscle  can  only  correct  low  degrees  of 
astigmatism;  hence  there  is  no  eyestrain. 

It  is  only  a  cycloplegic  that  will  reveal  this  uncon- 
scious correction;  hence  no  eyes  can  be  reported  as 
normal  unless  carefully  examined  under  atropine  or 
homatropine. 

The  removal  of  eyestrain  does  not,  in  the  strict  sense 
of  the  word,  cure  epilepsy  any  more  than  it  cures  a 
*  "  Nervous  System,"  vol.  ii.,  page  795. 


EYESTRAIN  167 

headache,  but  by  removing  the  eyestrain  we  remove 
one  of  the  causes,  and  frequently  the  only  cause,  that 
determines  an  attack.  An  epileptic  attack  and  some 
forms  of  headache  only  differ  in  degree;  they  are 
"  nerve-storms." 

The  foregoing  remarks  apply  equally  to  those  so-called 
choreiform  movements,  tics,  and  habit-spasms  which  are 
so  distressing  to  children  and  young  adults,  which  take 
the  form  of  spasmodic  involuntary  twitching  of  the 
facial  muscles. 

Constant  blinking  of  the  eyes  in  children  should 
always  make  one  suspicious  of  the  presence  of  eyestrain. 

In  all  such  cases  it  is  most  important  to  eliminate  eye- 
strain. Such  cases  are  purely  functional,  and  it  is  not 
too  much  to  say  that  a  pair  of  suitable  glasses  will 
immediately  alleviate,  and  not  infrequently  cause  a 
complete  cessation  of,  the  symptoms. 

Vertigo  as  a  symptom  of  peripheral  irritation  may 
exist,  but  its  commonest  manifestation  is  in  connection 
with  diplopia. 

Nausea  and  vomiting  have  already  been  mentioned 
in  connection  with  ocular  headache;  they  may  exist 
without  headache,  and  apart  from  diplopia  and  vertigo, 
and  may  be  due  to  eyestrain.  Their  presence  is  ex- 
plained by  the  intimate  association  of  the  fifth  with  the 
vagus. 

As  an  instance  of  the  variety  of  form  which  peripheral 
irritation,  through  eyestrain,  may  assume,  hyperhidrosis 
must  be  mentioned. 

3.  Nerve-Power  Waste;  Nerve-Exhaustion;  Neuras- 
thenia; Brain-Fag. — This  is  a  manifestation  of  eyestrain 
that  is  as  common  as  it  is  subtle.  Subtle  it  must  be, 
as  every  possible  cause  has  been  cited  as  the  origin  of 
the  various  groups  of  symptoms  which  are  exhibited, 
except  the  eyes.  It  has  not  been  sufficiently  recognized 
that  in  a  large  majority  of  those  cases  called  neuras- 
thenia the  real  trouble  is  a  constant   "  nerve-power 


1 68  THE  REFRACTION   OF  THE  EYE 

leakage  "  or  waste  of  nervous  energy,  and  in  a  large 
number  of  cases  eyestrain  is  the  cause. 

This  "  nerve-waste  "  may  exist  in  a  person  of  robust 
nervous  temperament  without  much,  if  any,  harm,  but 
in  one  whose  nervous  conditions  are  unstable  it  must 
in  time  show  itself.  Even  perfectly  robust  individuals 
showing  no  symptoms  of  eyestrain  may  manifest  it 
when  under  altered  conditions,  such  as  after  shock  of 
any  kind,  or  lowering  of  the  general  vitality  from  any 
cause.  ■ 

A  person  with  a  low  degree  of  astigmatism,  or  with 
anisometropia,  or  want  of  balance  of  the  ocular  muscles 
during  all  his  waking  hours  is  sending  down  impulses  to 
the  eye  to  correct  the  defect,  and  when  he  starts  on 
near  work  he  starts  with  a  big  deficit,  and  further  strain 
results.     This  must  mean  great  waste  of  nervous  energy. 

Writers  on  neurasthenia  agree  that  the  ages  when  the 
disease  is  most  liable  to  show  itself  are  between  20  and 
49,  and  these  are  exactly  the  ages  when  this  form  of 
eyestrain  is  most  manifest. 

Again,  those  most  affected  are  almost  invariably  people 
who  are  engaged  in  constant  near  work,  such  as  engine- 
fitters,  post-office  and  bank  clerks,  teachers,  journalists, 
and  professional  men  and  women. 

Insomnia  is  a  very  prominent  symptom  of  eyestrain, 
and  so  a  vicious  circle  is  started,  eyestrain  producing, 
among  other  troubles,  insomnia,  and  insomnia  in  its 
turn  aggravating  the  patient's  condition  because  the 
all-important  restorative  is  wanting.  The  extremely 
depressing  effect  of  insomnia  is  a  matter  of  common 
knowledge,  and  the  depression  caused  by  it  has  led 
hundreds  to  suicide. 

The  physician  who  is  called  upon  to  treat  a  so-called 
"  functional  nerve  disorder,"  and  fails  to  eliminate  the 
element  of  eyestrain,  fails  in  his  duty  both  to  himself 
and  to  his  patient,  for  there  is  no  functional  trouble  that 
may  not  be  due  to  eyestrain, 


EYESTRAIN  169 

The  depression  attending  "  nerve- waste  "  may  lead  to 
the  alcohoUc  habit,  and  the  irritabiHty  so  often  present 
in  those  suffering  from  functional  nerve  disorders  often 
induces  the  sufferer  to  resort  to  sedatives,  such  as 
morphia. 

If  the  "  nerve- waste  "  is  arrested,  the  depression  or  irri- 
tabihty  is  removed,  and  the  drug  habit  is  more  easily 
overcome. 

Once  we  allow  that  this  nerve-power  waste  may  exist, 
we  recognize  that  there  seems  to  be  no  limit  to  the 
various  ailments  which  may,  perhaps  gradually,  and 
often  imperceptibly,  follow. 

Dyspepsia  and  constipation,  and  other  disorders  of 
the  alimentary  tract,  may  be  caused  by  eyestrain.* 
The  dyspepsia  may  be,  as  we  have  seen,  a  direct  reflex 
irritation  from  the  strained  eyes  through  the  pneumo- 
gastric,  and  it  may  also  be  the  result  of  nerve-power 
waste;  for  if  this  waste  causes  depression,  and  the  due 
amount  of  nerve  energy  for  digestion  is  not  available, 
then  the  various  digestive  processes  are  interfered  with, 
not  only  in  the  stomach,  but  throughout  the  digestive 
tract. 

It  has  been  suggested  that  eyestrain  is  present  in 
patients  suspected  of  being  in  the  so-called  pretuber- 
culous  stage,  and  that  by  removing  the  eyestrain  the 
advent  of  tubercle  may  be  prevented. 

This  is  quite  consistent  with  the  above  theories,  and 
with  the  experience  of  many. 

The  patient  suffering  from  nerve-power  waste  has  a 
low  resisting  power  to  all  disease  germs,  and  is  in  what 
one  might  call  a  "  pre-germ  stage,"  so  that  he  is  more 
liable  to  any  infection  than  the  normal  person. 

There  can,  therefore,  be  little  doubt  that  the  correc- 
tion of  eyestrain  takes  a  very  prominent  place  in  pre- 
ventive medicine. 

*  "  Aberrant  Dyspepsias,"  by  Leonard  Williams.  {The 
fiospital,  Decernber  12,  1908.) 


170  THE   REFRACTION   OF  THE   EYE 

The  chief  reason  why  the  eyes  are  so  seldom  suspected 
of  being  the  cause  of  neurasthenia,  brain-fag,  and  the 
different  varieties  of  functional  nervous  troubles,  is  that 
the  majority  of  patients  have  either  good  sight  or  they 
are  already  wearing  glasses  which  apparently  correct 
their  refractive  errors. 

The  enormous  importance  of  correcting  eyestrain  has 
been  abundantly  shown  during  the  war  in  the  number 
of  soldiers  suffering  from  shell-shock,  and  neurasthenia 
following  on  head  injuries.  The  loss  of  nerve  energy 
consequent  on  an  accident  renders  it  imperative  to 
arrest  any  further  leakage,  and  the  correction  of  a  low 
error  of  astigmatism  or  anisometropia,  or,  in  bad  cases, 
where  testing  is  impossible,  the  bandaging  of  one  or 
both  eyes,  has  led  to  immediate  improvement  in  the 
whole  condition  of  the  sufferer.  {Note. — It  ought  never 
to  be  forgotten  that  credit  for  forcing  the  attention 
of  the  profession  to  the  importance  of  eyestrain  belongs 
to  Gould  of  America,  who  brought  out  his  fascinating 
Biographic  Clinics  in  1903.  Dr.  Harwood,  in  an 
admirable  paper  read  at  the  Oxford  Ophthalmological 
Congress  in  1917,  maintains  that  the  real  cause  of  the 
troubles  grouped  as  eyestrain,  is  an  instability  of  the 
normal  tonic  contraction  of  one  or  both  ciliary  muscles, 
and  that  this  instability  is  produced  when,  as  Weir 
Mitchell  puts  it,  the  brain  is  sensitized  by  disease.) 


CHAPTER  XIII 

HETEROPHORIA 

It  has  been  shown  (page  44)  that  in  ideal  binocular  vision 
the  visual  axes  are  parallel  when  the  eyes  are  at  rest, 
E  R  (Fig.  76,  A),  and  that  when  the  eyes  accommodate 
for  a  point  p,  both  eyes  converge  to  that  point;  that 
in  normal  vision,  if  we  destroy  the  possibility  of  fusion, 
that    convergence    lags    behind    accommodation,    and 


D  R 


R   D 


o  R 


Fig.  76. 

instead  of  converging  for  p,  the  visual  axes  are  in  the 
d  rection  e  a  (Fig.  76,  B),  the  difference  between  a  and 
p  being  the  "  fusion  supplement."  We  have  also  seen 
that  if  the  position  of  rest  is  one  of  divergence  (Fig.  76,  C), 
A  is  further  removed  from  p,  and  the  "  fusion  supple- 
ment "  is  larger.  Now,  this  position  of  divergence  at 
rest  is  caused  by  "  muscular  insufficiency,"  in  this  case 

of  the  internal  recti  muscles.    This  means  that  the 

171 


172  THE  REFRACTION  OF  THE  EYE 

internal  recti  are  insufficient  to  produce  parallelism 
without  active  muscular  contraction  which  the  demand 
for  binocular  vision  necessitates  during  all  the  waking 
hours;  hence  the  muscles  are  never  at  rest,  and  when 
the  necessity  for  convergence  arises,  the  interni  start 
with  a  deficit  of  power.  The  constant  using  up  of  part 
of  the  convergence  power  fatigues  the  internal  recti 
muscles,  and  the  positive  part  of  the  amplitude  of  con- 
vergence will  be  found  very  much  diminished. 

The  amplitude  of  convergence  may  be  quite  up  to  the 
average,  but  if  the  positive  part  of  it  is  too  small,  this 
indicates  the  presence  of  insufficiency  of  the  internal 
recti  and  the  liability  to  eyestrain.  Take  a  patient 
who  has  3  m.a.  of  latent  divergence,  and  whose  con- 
vergence near  point  is  14  cms. :  we  have — 

ca  =  7    -  (  -  3) 
=  7+3 
=  10  m.a. 

10  m.a.  of  amplitude  of  convergence  would  be  quite  suffi- 
cient if  it  were  all  positive,  but  only  7  are  positive,  and 
only  from  J  to  J  of  this  power  should  be  used  for  any 
length  of  time,  which  means  that  only  about  2  m.a. 
should  be  used,  which  would  be  useless  for  near  work. 
Remove  the  insufficiency — that  is,  remove  his  latent 
divergence — and  he  will  not  only  have  10  m.a.  of  positive 
convergence  power,  but  even  more,  for  the  constant 
fatigue  of  the  internal  recti,  produced  by  the  work  of 
overcoming  the  latent  divergence,  will  be  removed. 

"  Insufficiency  "  of  a  muscle  implies  that  the  muscle 
is  relatively  weaker  than  its  opponent,  so  that  in  static 
and  dynamic  vision  extra  impulses  have  to  pass  to  that 
muscle  in  order  to  produce  perfect  fusion. 

An  insufficient  muscle  is  not  necessarily  weak.  It  is 
insufficient  because  its  opponent  through  spasm  or  pre- 
ponderance {Uebergewicht,  as  Graefe  called  it)  is  "too 
sufficient";  hence  in  the  case  of  insufficiency  of  the 


HETEROPHORIA  173 

interni,  although  the  muscles  may  be  insufficient  for 
convergence,  they  may  be  perfectly  able  to  take  their 
part  in  the  associated  movements  of  the  eyes  to  either 
side. 

When  a  muscle  is  absolutely  weak,  it  is  wiser  to 
employ  the  term  "inefficiency."  Thus,  convergence 
inefficiency  means  inability  in  the  internal  recti  to  act 
normally,  irrespectively  of  their  opponents.  A  patient 
suffering  from  convergence  inefficiency  appears  to  have 
no  power  to  supply  a  fusion  supplement.* 

Insufficiency  may  be  present  in  any  of  the  muscles. 
If  the  visual  axes  at  rest  are  convergent,  the  external 
recti  muscles  are  insufficient  to  produce  paralleUsm 
(Fig.  76,  D).  In  this  case  binocular  distant  vision, 
which  should  be  perfect  rest  to  the  eyes,  necessitates  the 
constant  contraction  of  the  external  recti,  and  they  are 
liable  to  become  fatigued  and  to  cause  eyestrain.  During 
convergence,  also,  we  may  find  latent  convergence,  the 
eyes  converging  to  a  point  nearer  than  p  (Fig.  76,  D), 
and  the  necessity  for  fusion  demands  contraction  of  the 
external  recti  to  overcome  this  overaction  of  the  interni : 
hence  fatigue.  And  so  with  the  other  muscles.  If  one 
muscle  by  spasm  or  preponderance  over  its  opponent 
prevents  the  eyes  from  assuming  the  normal  position 
when  at  rest,  there  is  liability  to  fatigue  and  eyestrain; 
whether  it  manifests  itself  or  not  (there  are  a  large 
number  of  cases  of  insufficiency  which  never  produce 
any  symptoms)  depends  entirely  on  the  amount  of 
insufficiency  and  the  nervous  condition  of  the  indi- 
vidual. 

When  we  consider  that  on  the  relative  strength  of  the 
muscles  of  the  eyeball  depends  the  position  of  the  eye, 
and  that  the  smallest  amount  of  preponderating  strength 
or  the  slightest  amount  of  weakness  of  one  muscle  will 

*  Landolt  calls  the  eyestrain  produced  by  the  first  variety 
(insufficiency)  "  peripheral  motor  asthenopia, "  and  that  produced 
by  the  second  variety  (inefficiency)  "  central  motor  asthenopia." 


174  THE  REFRACTION   OF  THE   EYE 

cause  a  displacement  or  a  tendency  to  displacement 
{i.e.,  a  latent  deviation)  of  the  eyes,  we  can  only  wonder 
that  the  condition  of  parallelism  of  the  visual  axes  in 
distant  vision  is  so  constantly  found.  The  secret  is,  that 
the  desire  for  binocular  vision,  obtained  by  the  fusion  of 
the  two  images,  acts  as  an  unconscious  stimulus  to  the 
weaker  muscle,  and  masks  the  relative  weakness.  If 
binocular  vision  be  impossible  through  the  sight  of  one 
eye  being  very  much  inferior,  then  the  stimulus  is  absent, 
and  the  eyes  assume  a  divergent  or  convergent  position 
of  rest,  which  becomes  manifest,  and  is  then  a  squint. 
In  other  words,  the  heterophoria  passes  into  a  hetero- 
tropia.  We  must  be  careful  not  to  use  the  term  "  in- 
sufficiency "  in  connection  with  squint;  it  is  quite 
unnecessary,  and  causes  a  great  deal  of  confusion  to 
apply  the  term  to,  for  instance,  an  atrophied  internal 
rectus  in  an  old  divergent  strabismus.  An  insufficiency, 
if  unrelieved,  may  pass  into  a  squint,  but  they  are  two 
distinct  things. 

In  estimating  insufficiency  of  a  muscle  we  must 
beware  of  attaching  too  much  importance  to  one  ex- 
amination; a  muscle  may  be  insufficient  at  one  time 
and  not  at  another. 

The  following  terms,  suggested  by  Stevens  of  New 
York,  are  now  in  common  use  in  designating  the  different 
forms  of  insufficiency  (see  plate,  page  45). 

Orthophoria  =  visual  axes  parallel,  and  lying  in  the 
same  horizontal  plane. 

Heterophoria  =  visual  axes  not  parallel  or  not  in  the 
same  horizontal  plane;  divided  into: 

1.  Exophoria.    The  eyes  tend  to  turn  out;  insuffi- 

ciency of  the  interni. 

2.  Esophoria.      The  eyes  tend  to  turn  in:  insuffi- 

ciency of  the  externi. 
3-  Hyperphoria.     One  eye  tends  to  be  on  a  higher 
level  than  the  other,  due  to  insufficiency  of  the 
superior  or  inferior  rectus. 


EXOPHORIA  175 

4.  Insufficiency  of  the  oblique  muscles — 

(a)  Hyperesophoria,  a  tendency  up  and  in. 
{b)  Hyperexophoria,  a  tendency  up  and  out. 

5.  Cyclophoria. 

I.  Exophoria  {Insufficiency  of  the  Internal  Recti — Con- 
vergence Strain). — "  Convergence  Insuflaciency  "  is  a 
latent  external  squint,  overcome  for  the  time  by  the 
strong  desire  for  single  vision. 

Strain  of  the  internal  recti  is  essentially  dependent 
upon  binocular  vision,  and  persons  who  have  not  the 
advantage  of  binocular  vision,  by  a  compensation  of 
Nature  cannot  suffer  from  this  trouble. 

Tests  for  Insufficiency  of  the  Interni. — It  has  been 
shown  that  the  Maddox  test  is  the  best  and  simplest. 
If  we  find  latent  divergence  for  distance,  or  latent 
divergence  of  more  than  a  metre  angle  at  J  metre,  or 
both,  we  can  positively  assert  that  the  interni  are 
insufficient,  and  we  can  confirm  this  by  ascertaining  the 
amplitude  of  convergence,  the  ordinary  working  distance 
of  the  patient,  and  the  reserve  power  of  convergence. 

Except  in  neurasthenic  insufficiency,  the  estimation  of 
the  adducting  power  of  the  interni  by  prisms  is  not  of 
much  use.  We  may  get  an  adducting  power  of  30°,  and 
yet  if  the  externi  are  "  preponderating  "  we  shall  have 
insufficiency  present. 

Although  exophoria  may  be  associated  with  uncor- 
rected high  hyperopia,  due  to  the  patient  approaching 
his  work  very  near  the  eyes  in  order  to  obtain  large 
retinal  images,  it  is  in  myopia  that  we  generally  find 
this  condition,  where  the  error  is  uncorrected,  or  only 
partially  corrected.  The  excess  of  convergence  over 
accommodation,  and  also  the  excessive  convergence, 
must  sooner  or  later  cause  fatigue  of  the  internal  recti. 

Treatment. — In  the  majority  of  cases,  when  the  want 
of  muscle  balance  is  small,  the  correction  of  the  error 
and  the  constant  wearing  of  the  glasses  will  in  a  short  time 


176  THE  REFRACTION  OF  THE  EYE 

remove  the  exophoria.  One  of  the  advantages  of  wear- 
ing the  concave  glasses  for  near  work  is  that  the  work 
must  be  held  some  distance  from  the  eyes,  which  of 
course  means  less  strain  to  the  convergence ;  and  as  this 
is  associated  with  a  restoration  of  the  harmony  between 
the  accommodation  and  convergence,  which  always 
means  less  strain,  one  is  not  surprised  at  the  good  result 
of  this  treatment. 

If  exophoria  still  persists  after  some  months  of  this 
treatment,  a  prism,  base  in,  should  be  prescribed  with 
the  glasses,  or,  if  the  concave  glass  is  fairly  strong,  the 
prism  effect  may  be  obtained  by  decentring  the  glass 
out  (see  Fig.  77). 

When  prisms  are  ordered,  it  is  unwise  to  give  the  full 
correction.  Suppose,  for  instance,  our  patient  shows 
an  exophoria  with  the  Maddox  distance  test  which  is 
corrected  by  a  4°  prism,  base  in,  before  one  eye,  if  we 
give  a  2°  prism,  base  in,  before  each  eye,  we  shall  be 
helping  the  muscles  too  much,  and  give  the  patient  no 
chance  of  improving;  we  should  in  this  case  prescribe 
a  1°  prism,  base  in,  before  each  eye.* 

It  should  never  be  forgotten  that  the  prism  treatment 
is  not  a  curative  treatment ;  we  are  treating  the  symptom, 
and  not  helping  the  condition  to  disappear. 

Even  when  prisms  are  ordered,  the  patient  should  be 
enjoined  to  use  them  as  special  glasses  to  be  worn  only 
for  NEAR  work. 

2.  Esophoria  (Insufficiency  of  the  External  Recti). — 
When  latent  convergence  is  demonstrated  for  distance, 
we  say  that  we  have  "  insufficiency  "  of  the  external 
recti- — that  is,  in  the  position  of  rest  the  externi  are 
relatively  weak.  If  there  be  no  manifest  convergence, 
the  visual  axes  assume  a  parallel  condition;  but  to 
maintain  this,  constant  active  contraction  of  the  external 

*  The  use  of  prisms  is  limited  to  about  4°  in  front  of  each 
eye,  as  any  stronger  prism  would  make  the  glasses  too 
heavy. 


ESOPHORIA 


177 


recti  must  take  place  when  distant  vision  is  used,  in 
order  to  prevent  diplopia.  Latent  convergence  is  very 
common,  but,  in  the  majority  of  cases,  gives  rise  to  no 
symptoms.  The  explanation  is,  that  the  latent  devia- 
tion is  slight,  and  in  our  civiHzed  state  active  use  of  the 


Fig.  77. 
P,  Object  looked  at ;  P',  apparent  position. 


eyes  is  mostly  associated  with  a  necessary  convergent 
condition.  When  the  latent  convergence  is  excessive, 
we  may  get  symptoms  of  eyestrain. 

Test  for  Insufficiency  of  the  Externi. — ^When  the 
Maddox  test  reveals  latent  convergence  for  distance  or 
for  J  metre,  it  is  present. 

12 


178  THE   REFRACTION   OF  THE   EYE 

The  abducting  power  of  the  normal  extern!  is  equal 
to  a  prism  of  7°  or  8°. 

Insufficiency  of  the  externi  is  generally  associated  with 
hyperopia. 

We  have  seen  that  in  hyperopia,  accommodation  is 
required  in  excess  of  convergence,  and,  unless  the  two 
can  be  dissociated,  the  hyperope  must  converge  to  a 
nearer  point  than  is  necessary.  This  over-contraction  of 
the  interni  causes  latent  convergence  for  distance,  and 
also  for  the  near  point  (Fig.  76,  D),  the  external  recti 
become  insufficient,  and  extra  impulses  must  pass  down 
to  these  muscles  in  order  to  obtain  "  fusion  vision." 
This  may  lead  to  fatigue  and  eyestrain,  which,  how- 
ever, often  disappear,  owing  to  the  development  of  a 
convergent  squint  and  loss  of  binocular  vision  (see 
page  91). 

Treatment. — By  putting  the  hyperope  into  glasses  we 
re-establish  the  harmony  between  convergence  and 
accommodation,  remove  the  spasm  of  the  internal  recti, 
and  consequently  also  the  insufficiency  of  the  externi, 
restoring  the  balance  of  all  the  muscles. 

One  of  the  pleasantest  things  to  do  in  ophthalmic 
work  is  to  cure  a  squint,  or  a  tendency  to  squint,  by 
simply  giving  the  patient  glasses.  If  parents  would 
only  realize  that,  in  a  large  number  of  cases,  this  can  be 
done  by  bringing  the  child  early  enough — i.e.,  before 
the  latent  squint  has  become  manifest,  or  possibly  during 
the  early  period  of  the  manifest  squint — there  would  be 
fewer  squints. 

It  is  very  seldom  that  we  have  to  resort  to  prisms  in 
the  treatment  of  esophoria,  but,  should  it  be  found 
necessary,  the  prism  must  be  placed  base  out,  and  if 
decentring  is  substituted,  convex  glasses  must  be 
decentred  outwards  (see  Fig.  78).     (See  page  208.) 

Note. — Method  of  finding  the  Amount  of  Decentring  necessary  to 
produce  the  Effect  of  a  given  Prism  in  a  given  Lens  (Ward  Holden) . 
— Take  8-7  mm.  as  the  distance  a  lens  of  i  d  most  be  moved 
to  produce  the  effect  of  a  prism  1°,  as  the  unit,  multiply  8*7  mm. 


HYPERPHORIA  I79 

by  the  number  of  the  prism  whose  effect  is  required,  and  divide 
the  product  by  the  number  of  the  lens  in  dioptres.  Thus  the 
effect  of  a  prism  3°  in  a  lens  of  7  d  is  obtained  by  decentring  that 

lens  to  the  extent  of  —  mm.  =.V7  mm. 

3.  Hyperphoria  {Insufficiency  of  the  Superior  or 
Inferior  Rectus). — This  condition  is  revealed  by  the 
Maddox  test  with  the  glass  rod  vertical  (see  page  48). 


Fig.  78. 
P,  Object  looked  at ;  P',  apparent  position. 


Treatment. — A  small  amount  is  often  present  where  an 
error  of  refraction  is  present,  and  generally  disappears 
in  a  few  weeks,  after  correction.  It  is  generally  asso- 
ciated with  astigmatism  and  anisometropia. 


l80  THE   REFRACTION   OF  THE   EYE 

If  prisms  have  to  be  resorted  to,  they  must  be  placed 
base  up  or  down,  according  to  the  condition  existing. 

A  hyperphoria  is  often  induced  when  reading  through 
glasses  correcting  anisometropia;  for  the  treatment  see 
page  158. 

4.  Hyperexophoria — Hyperesophoria  (Insufficiency  of 
the  Oblique  Muscles). — This  is  the  result  of  the  superior 
oblique  of  either  eye  being  too  strong  for  its  inferior, 
or  vice  versa.  The  parallelism  of  the  vertical  meridians 
of  the  corneae  is  maintained  by  the  equilibrium  of  these 
muscles;  hence,  when  one  muscle  is  weaker,  excessive 
work  must  be  put  upon  it  in  order  to  preserve  the 
parallelism,  and  eyestrain  results. 

The  Maddox  rod  test  reveals  the  heterophoria. 

We  must  hope  that  the  correction  of  the  refractive 
error  will  remove  the  want  of  balance,  and,  if  necessary, 
we  must  use  prisms  set  obliquely;  but  some  cases,  un- 
fortunately, appear  to  be  incurable. 

5.  Cyclophoria. — This  is  a  rare  form  of  heterophoria, 
and  is  due  to  the  turning  of  one  or  both  eyes  round  an 
antero-posterior  axis.  If  the  Maddox  rods  in  a  frame 
are  placed  exactly  horizontal  before  one  eye,  and  the 
streak  of  light  is  seen  by  the  patient  as  being  more  or 
less  sloping,  instead  of  perfectly  vertical,  cyclophoria 
is  present.  When  present,  it  is  generally  found  asso- 
ciated with  oblique  astigmatism.  (See  Maddox, 
"  Ocular  Muscles,"  1907,  page  236.) 

Treatment. — Extreme  care  should  be  taken  to  correct 
completely  the  errors  of  refraction,  and  very  special  care 
should  be  taken  in  ascertaining  the  exact  angle  for  the 
cylinder,  and  also  in  ascertaining  that  the  optician  has 
rigidly  carried  out  instructions.  As  a  rule,  this  suffices 
to  remove  the  trouble.     Prisms  are  of  no  use. 

Heterophoria  in  Emmetropia. — Although  rare,  this 
condition  may  exist,  and  may  be  due  to — 

I.  Excessive  or  prolonged  convergence. 

This  generally  produces  at  first  an  esophoria,  probably 


HETEROPHORIA  l8l 

due  to  slight  spasm  of  the  internal  recti,  and  later, 
when  fatigue  sets  in,  an  exophoria.  It  is  difficult  to 
say  whether  this  form  of  heterophoria  produces  any  defi- 
nite symptoms,  as  the  eyestrain  that  may  be  present  is 
more  likely  due  to  the  strain  of  the  accommodation. 

2.  Weakness  of  certain  muscles  produces  "  ineffi- 
ciency," caused  by  general  debiUty,  especially  noticed  in 
those  recovering  from  a  severe  illness. 

3.  Congenital  defect.  One  of  the  external  muscles 
may  be  attached  to  the  eyeball  too  far  forward,  or  its 
opponent  too  far  back. 

Treatment. — In  the  healthy,  rest  of  the  eyes  is  the 
obvious  treatment,  if  symptoms  of  eyestrain  appear; 
and  if  an  excessive  amount  of  fine  near  work  (such  as 
miniature  painting)  has  to  be  done,  short  periods  of 
work  should  alternate  with  outdoor  exercise  and  some 
other  form  of  rest  to  the  eyes. 

When  the  heterophoria  is  the  result  of  defective  or 
enfeebled  muscles,  we  must  first  of  all  improve  the 
general  health  by  enjoining  outdoor  exercise  and  atten- 
tion to  the  bowels,  and  by  the  administration  of  tonics, 
and  then,  remembering  that  absolute  rest  will  only  tend 
to  increase  the  weakness,  we  must  commence  regular 
exercise  of  convergence  (if  we  are  dealing  with  "  con- 
vergence insufficiency ")  for  short  periods,  gradually 
increased,  and  forbid  the  patient  to  use  the  eyes  for 
near  w^ork  except  at  these  times.  To  carry  out  this 
treatment  in  young  subjects  the  use  of  a  cycloplegic  is 
most  helpful.  Forced  or  prolonged  efforts  of  conver- 
gence would  only  help  to  increase  the  fatigue  of  the 
internal  recti,  and  therefore  we  must  commence  with 
very  short  efforts,  and  increase  those  efforts  very  slowly. 

Orthoptic  training  or  gymnastic  exercises  can  be,  with 
very  great  benefit,  extended  to  the  extrinsic  muscles. 
For  strengthening  the  internal  recti  we  employ  properly 
regulated  convergence  for  a  short  time  at  different 
periods  of  the  day,  the  time  to  be  gradually  increased 


1 82  THE  REFRACTION   OF  THE  EYE 

as  the  muscles  increase  in  strength,  measured,  of  course, 
by  the  tests  previously  cited.  For  strengthening  the 
external  recti  we  must  employ  prisms  with  their  base 
in.  The  best  plan  is  to  provide  the  patient  with  a 
square  prism — say  2° — and  tell  him  to  practise  fusion 
several  times  a  day ;  and  when  this  is  accomplished  with 
ease,  we  can  gradually  increase  the  strength  of  the  prism 
until  the  "  insufficiency  "  disappears. 

As  a  last  resource,  weak  prisms,  with  their  base  in, 
may  be  ordered  for  convergence  insufficiency.  With  their 
help  the  convergence  effort  is  diminished,  but  all  hope 
of  the  muscles  regaining  their  normal  condition  must 
be  abandoned,  unless  the  prisms  are  only  used  tem- 
porarily while  the  general  condition  is  being  improved. 
Prisms  employed  in  this  way  never  cure  "  insufficiency  " ; 
they  only  relieve  it. 

Treatment  by  Tenotomy. — In  a  few  special  cases 
tenotomy  has  succeeded  when  the  milder  treatment  has 
failed.  The  cases  where  it  is  likely  to  be  beneficial 
are  patients  with  marked  exophoria  possessing  an  ample 
range  of  convergence.  A  tenotomy  of  one  or  both 
external  recti,  and  in  bad  cases  advancement  of  an 
internal  rectus,  will  remove  the  exophoria;  and  by  this 
setting  free  the  whole  of  the  convergence  power,  by 
turning  the  "  negative  "  convergence  into  "  positive," 
the  symptoms  of  eyestrain  are  removed. 

Landolt  gives  a  very  good  example  of  such  a  case 
(Fig.  79,  a).  Before  the  operation  there  was  3  m.a.  of 
divergence,  and  the  convergence  near  point  was  14  cms. 
with  eyestrain.     In  this  case 

ca  =  Y/  -  (  -3)  =  Y/-  +  3  =  10  m.a., 

but  only  seven  of  these  lo-metre  angles  of  convergence 
power  were  available;  tenotomy  of  the  external  rectus 
removed  the  divergence,  the  whole  of  the  10  m.a.  became 
available,  and  the  eyestrain  disappeared  (Fig.  79,  a'). 
Fig.  79,  h,  illustrates  another  case  which  was  cured 


MALINGERING  183 

by  tenotomy;  here  the  result  was  not  so  perfect  as  in 
the  first  case  (Fig.  79,  a). 

Fig.  79,  c,  is  an  example  of  what  Landolt  calls  neuras- 


FiG.  79. 

thenic  insufficiency,  when  the  amplitude  of  convergence 
is  exceedingly  small,  and  when  tenotomy  can  do  no  good. 

Malingering. — A  malingerer  will  sometimes  pretend  that  one 
eye  is  blind,  in  order  to  escape  war  service,  or  to  obtain  compen- 
sation after  injury.  Malingering  is  sometimes  a  symptom  of 
hysteria.     It  may  be  detected  by — 

1.  The  Prism  Test. — Both  eyes  being  uncovered,  and  directed 
to  a  distant  object,  a  prism,  base  up  or  down,  is  placed  before 
the  "  blind  "  eye.  Diplopia  is  complained  of  if  the  eye  is  not 
blind,  and  the  fraud  is  exposed. 

If  a  6°  prism  is  held  before  the  "  blind  "  eye,  base  out,  this  eye 
will  move  in,  at  the  moment  of  placing  the  prism,  in  order  to 
avoid  diplopia,  if  it  is  not  blind. 

2.  The  "Friend  "  Test  (see  page  153) . — The  malingerer  does  not 
know  that  with  this  test  he  can  read  only  half  the  letters  with 
one  eye.     If  he  reads  the  whole  word,  he  is  detected. 

3.  Bishop  Harman's  Diaphragm  Test. — 'This  instrument  is  not 
only  useful  for  detecting  malingering,  but  is  also  a  very  ready 
test  of  binocular  vision  and  its  defects.  (For  description,  see 
The  Ophthalmoscope,  vol.  viii.,  p.  495.) 


CHAPTER  XIV 

STRABISMUS 

Squint. — A  heterophoria,  or  latent  squint,  may  at  any 
time  become  heterotropia,  or  manifest  squint,  if  the 
necessary  muscular  effort  to  preserve  parallelism  cannot 
be  made  or  maintained.  Thus,  insufficiency  of  the 
internal  recti  in  myopia,  produced  by  excessive  con- 
vergence, may  become  temporarily  or  permanently  a 
divergent  squint,  and  we  have  already  seen  how  a  con- 
vergent squint  develops  when  the  patient  cannot  use  his 
accommodation  in  excess  of  his  convergence  in  hyper- 
opia (page  91). 

Varieties  of  Squint. — (i)  Concomitant,  in  which  the 
squinting  eye  moves  with  its  fellow,  and  always  deviates 
to  the  same  degree  from  the  correct  position. 

(2)  Paralytic,  when  the  movement  of  the  squinting 
eye  is  restricted  by  paralysis  of  the  muscle. 

We  shall  deal  in  these  pages  only  with  the  first  variety. 

Forms  of  Concomitant  Squint — (i)  Convergent. — In- 
ternal squint  (esotropia). 

If  the  squinting  eye  is  not  amblyopic,  there  is  homony- 
mous diplopia.  In  Fig.  80,  r,  the  right  eye  is  fixing  the 
object  o,  but  l,  the  left  eye,  which  is  squinting  in,  re- 
ceives the  image  of  o  at  m',  which  is  on  the  nasal  side 
of  the  macula  m;  hence  the  left  eye  projects  o  to  the 
left  or  same  side. 

2.  Divergent  strabismus. — External  squint  (exotropia) , 

If  the  squinting  eye  is  not  amblyopic,  there  is  heterony- 
mous or  crossed  diplopia.     In  Fig.  81,  R,  the  right  eye,  is 

184 


STRABISMUS  185 

fixing  the  object  o,  but  l,  the  left  eye,  is  squinting  out, 
and  receives  the  image  of  o  at  m',  which  is  on  the  tem- 
poral side  of  the  macula  m;  hence  L  projects  o  to  the 
right  or  opposite  side. 


Fig.  80. — Homonymous  Diplopia. 

3.  Vertical  strabismus  (hypertropia),  in  which  the 
visual  axis  of  one  eye  is  deviated  upwards. 

These  three  different  forms  of  concomitant  squint 
may  be — 

1.  Constant,  in  which  one  eye  is  always  the  squinting 
eye.     This  condition  is  also  called  monolateral. 

2.  Alternating,  in  which  either  eye  can  fix,  the  fellow 


•86 


THE  REFRACTION   OF  THE  EYE 


squinting;  in  these  cases  the  vision  of  both  eyes  is 
generally  equally  good. 

Squints  may  also  be  periodic  or  intermittent.  When 
the  squint  is  developing — for  instance,  when  the  hetero- 
phoria  is  passing  into  a  heterotropia — the  latter  may  be 


M'    M  M 

Fig.  8i. — Heteronymous  Diplopia. 


manifested  only  after  fatigue,  or  when  certain  constitu- 
tional conditions  are  present. 

Maddox  puts  the  case  very  clearly,  thus:  When  the 
breadth  of  diplopia  is  greater  than  the  breadth  of  fusion 
power,  no  effort  can  unite  the  images,  and  this  is  (i)  a 
constant  squint.  When  they  are  almost  equal,  the 
images  may  be  united  by  a  great  effort  for  a  short  time ; 


STRABISMUS  187 

this  is  (2)  a  periodic  squint.  When  the  breadth  of 
diplopia  is  considerably  less  than  the  breadth  of  fusion 
power,  the  images  are  easily  united,  and  this  is  (3)  a 
latent  squint  (heterophoria) .  The  difference  between 
the  three  is  merely  a  question  of  degree. 

Angle  Gamma. — We  must  be  careful  to  distinguish  between  a 
real  squint  and  an  apparent  squint.  We  may  have  in  hyperopia 
an  apparent  divergent  squint,  and  in  myopia  an  apparent  con- 
vergent squint,  due  to  the  visual  and  optic  axes  not  coinciding. 

When  the  optic  axis  passes  through  the  fovea  it  coincides  with 
the  line  of  vision  or  line  of  sight,  and  also  with  the  line  of  fixation ; 
but  the  exception  to  this  is  the  rule,  and  an  angle  is  formed  by 
the  line  of  fixation  m  o  with  the  axis  a  a'  (Fig.  82).  This  angle 
is  called  the  angle  gamma  (o  m  a).  (The  angle  oka  [Fig.  82] 
made  by  the  line  of  vision  and  the  optic  axis  may  be  considered 
identical  with  the  angle  o  m  a,  and  is  sometimes  called  the  angle 
gamma.) 

The  angle  gamma  is  positive,  as  in  Fig.  82,  when  the  fovea  is 
on  the  outer  side  of  the  optic  axis,  and  it  is  generally  positive  in 
emmetropia  and  hyperopia,  and  in  some  cases  of  hyperopia  the 
angle  is  so  great  (amounting  even  to  10°)  that  it  gives  the  eyes 
an  appearance  of  divergence  (see  Fig.  83) — an  apparent  divergent 
squint;  the  eyes,  although  looking  at  and  fixing  the  point  o, 
appear  divergent  in  the  direction  a  o'. 

The  angle  gamma  is  negative  when  the  fovea  (f,  Fig.  84)  is  on 
the  inner  side  of  the  optic  axis — that  is,  between  the  optic  axis 
and  the  optic  nerve.  In  some  cases  of  myopia  this  is  so  marked 
as  to  give  the  eyes  the  appearance  of  convergence  (see  Fig.  84) — 
an  apparent  convergent  squint ;  the  eyes,  although  looking  at  the 
point  o,  appear  to  converge  in  the  direction  of  a  o'. 

The  angle  alpha  (o  x  e.  Fig.  82)  is  the  angle  formed  by  the  axis 
which  passes  through  the  most  curved  part  of  the  cornea  (the 
summit)  with  the  line  of  vision.  It  is  spoken  of  as  positive  when, 
as  in  Fig.  82,  the  anterior  portion  of  the  corneal  axis  is  situated 
on  the  outer  side  of  the  line  of  vision,  and  negative  when  it  is  on 
the  inner  side.  Generally  the  axis  of  the  cornea  very  nearly 
coincides  with  the  optic  axis,  so  that  for  all  practical  purposes 
the  angles  gamma  and  alpha  mean  the  same  thing. 

The  etiology  of  Concomitant  Squint.— Any  cause 
which  disturbs  the  muscular  equilibrium  may  be  the 
starting-point  of  a  squint,  so  that  any  of  the  causes  of 
heterophoria  (see  page  174)  may  be  the  factors  in  the 
causation  of  a  squint;  but  the  chief  are — 

I.  The  Accommodation  Theory. — We  have  already  seen 
how,   in  hyperopia   (page  91),   when  the  patient  has 


i88 


THE  REFRACTION  OF  THE  EYE 


Fig.  82. — A  Schematic  Figure  to  show  Angles  a  and    7. 
(After  Landolt.) 

A  a',  Optic  axis;  k,  nodal  point;  m,  centre  of  rotation;  c,  centre 
of  cornea;  b  b,  base  of  cornea;  e  l,  corneal  axis;  f,  fovea 
centralis;  o,  point  of  fixation;  k  o,  line  of  vision;  m  o, 
line  of  fixation ;  o  x  e,  angle  a ;  o  m  a,  angle  -y. 


STRABISMUS 


189 


Fig,  83. — Apparent  Divergent  Strabismus  due  to  a  Large 
Positive  Angle  Gamma. 


Fig.  84. — Apparent  Convergent  Strabismus  due  to  a 
Negative  Angle  Gamma. 


I  go  THE   REFRACTION   OF  THE   EYE 

to  use  his  accommodation  in  excess  of  his  convergence, 
a  convergent  squint  is  developed,  if  he  cannot  dissociate 
the  two;  he  has  to  choose  between  indistinct  binocular 
vision  and  clear  monocular  vision  with  a  squint,  and  he 
chooses  the  latter. 

Again,  in  myopia  he  has  to  use  his  convergence  in 
excess  of  his  accommodation;  "insufficiency"  of  the 
interni  develops,  and  in  time  an  external  squint  is 
manifest. 

2.  Anatomical  Peculiarities — The  Muscle  Theory. — A 
broad  face  with  a  large  interpupillary  distance  means 
the  necessity  for  greater  convergence;  a  narrow  orbit 
with  a  long  myopic  eye  prevents  freedom  of  movement 
of  the  eye,  and  consequently  causes  greater  strain  and 
the  necessity  for  greater  muscular  effort  in  convergence. 
The  external  rectus  may  be  inserted  into  the  eye  too 
far  forward  or  the  internal  rectus  too  far  back,  or  vice 
versa. 

Any  of  these  conditions,  especially  if  associated  with 
ametropia,  may  be  strong  factors  in  the  causation  of  a 
squint. 

3.  Non-Development  of  the  Fusion  Sense. — At  birth 
the  eyes  move  independently  of  each  other,  and  hence 
new-born  children  often  squint.  As  they  begin  to  take 
notice  of  surrounding  objects  they  develop  the  power 
of  fusion,  and  the  two  images  which  fall  on  the  maculse 
are  thus  fused  by  the  brain,  the  centre  being  known  as 
the  "  fusion  centre,"  or  centre  for  binocular  vision. 

Any  cause  which  reduces  the  visual  acuity  of  one  eye 
tends  to  develop  a  squint,  especially  if  a  heterophoria  or 
latent  disturbance  of  equilibrium  pre-exist.  Among  the 
commonest  causes  may  be  mentioned  corneal  opacities, 
cataract,  and  intra-ocular  diseases.  Thus,  a  patient 
with  heterophoria  has  an  attack  of  keratitis  in  one  eye ; 
corneal  opacities  result,  which  lower  the  visual  acuity  of 
this  eye,  and  a  manifest  squint  develops. 

The  amblyopia  may  be  congenital. 


STRABISMUS 


191 


Note. — This  amblyopia,  which  causes,  or  helps  to 
cause,  a  squint,  must  not  be  confused  with  the  ambly- 
opia which  is  the  result  of  the  squint,  and  which  develops 
through  the  non-use  of  the  eye.  When  a  squint 
develops,  diplopia  must  occur  at  first,  and  t )  g  t  rid  of 


A 

Ppimany  PoaitLO/i 


N     5 


Screen 


B 

5econdariy     PoftiCion 
Fig.  85. 


this  diplopia,  the  brain  refuses  to  recognize  the  image 
from  the  squinting  eye;  this  form  is  called  amblyopia 
ex  anopsia. 

Worth  maintains  that  the  essential  cause  of  squint  is  a 
defect  of  the  fusion  faculty,  and  that  this  defect  alone 
is  sufficient,  even  if  there  be  no  error  of  refraction. 


192  THE   REFRACTION   OF  THE   EYE 

The  Diagnosis  and  Measurement  of  Concomitant 
Squint. — Let  us  proceed  to  measure  a  convergent  squint. 

Make  the  patient  fix  an  object,  say,  a  couple  of  metres 
from  the  eyes,  taking  care  to  place  the  object  midway 
between  the  two  eyes.  Let  us  suppose  that  the  left  eye 
fixes  the  object  and  the  right  eye  squints  inwards:  we 
note  the  external  margin  of  the  cornea  of  both  eyes  by 
making  a  small  ink  spot  on  the  lower  lid;  let  s  be  this 
mark  on  the  right  eye  (Fig.  85)  and  n'  on  the  left.  We 
now  cover  the  left  eye  with  a  screen,  and  tell  the  patient 
to  fix  the  object  again;  this  he  does  with  the  right  eye, 
and  we  notice  a  marked  excursion  of  this  eye;  we  now 
note  the  position  of  the  external  margin  of  the  cornea  N. 
The  distance  n  s  is  the  primary  deviation. 

When  the  left  eye  is  covered  and  the  patient  is  fixing 
with  the  right  eye,  if  we  look  behind  the  screen  we 
notice  that  the  left  eye  makes  a  distinct  incursion ;  and  if 
we  mark  on  the  lid  the  two  positions  of  this  eye,  we  get 
the  distance  s'  n'  as  representing  the  secondary  devia- 
tion, which  is  equal  to  the  primary  deviation.  This 
enables  us  to  diagnose  very  easily  between  a  concomitant 
and  a  paralytic  squint,  for  in  the  latter  the  secondary 
deviation  is  always  very  much  greater  than  the  primary. 
We  measure  a  divergent  squint  in  the  same  manner. 

When  the  squinting  eye  is  blind,  we  must  use  a 
strabismometer  (Fig.  86),  and  read  off  on  the  scale  the 
amount  of  the  squint  in  millimetres. 

A  much  more  reliable  method  is  to  measure  the 
amount  of  squint  by  the  perimeter  (Fig.  87).  Placing 
the  patient  so  that  the  squinting  eye  is  opposite  the 
fixation-point  f,  and  with  both  eyes  uncovered,  we 
direct  him  to  look  at  a  distant  point  d,  the  squinting 
eye  and  f  and  d  being  all  in  one  line.  We  now  hold  a 
small  candle  or  match  at  the  fixation-point  F,  and 
gradually  move  it  along  the  arc,  which  is  placed  hori- 
zontally, looking  directly  behind  the  candle  for  its 
image  reflected  on  the  cornea  of  the  squinting  eye. 


STRABISMUS  1 93 

When  the  image  is  in  the  centre  of  the  pupil  we  read 
off  the  position  of  the  candle  on  the  arc,  and  the  degree 
mark  represents  the  angle  of  the  strabismus. 
Treatment. — 

1.  The  correction  of  the  error  of  refraction. 

2.  The  education  of  the  fusion  sense. 

3.  The  education  of  the  amblyopic  eye. 

4.  The  readjustment  of  the  muscles  by  operation. 

Convergent    Strabismus. — Almost    80    per    cent,    of 
patients  suffering  from  convergent  strabismus  are  hyper- 


FiG.  86. 

opes,  and  the  defect  is  manifested  very  early  in  life — 
in  fact,  when  the  child  begins  to  use  his  eyes  for  near 
vision,  looking  at  picture-books,  etc.;  the  majority  of 
such  patients  develop  a  squint  about  the  age  of  three. 

The  squint,  as  a  rule,  develops  slowly,  and  the  parents 
are  not  the  first  to  notice  it.  They  generally  assign  as 
the  cause  an  illness,  such  as  measles,  or  the  imitation  of 
a  squinting  companion. 

The  first  treatment  is  to  put  the  eyes  under  atropine 
for  at  least  a  week,  and  in  slight  cases  the  squint  entirely 

13 


194 


THE  REFRACTION  OF  THE  EYE 


disappears  when  the  eyes  are  fully  under  a  cyclop' egic. 
The  refraction  is  then  estimated  by  retinoscopy,  and  the 


full  correction,  less  i  d,  is  ordered  in  large  oval  or 
circular  spectacles,  to  be  worn  always.  For  instance, 
suppose  under  atropine   retinoscopy  at  i  metre  gives 


STRABISMUS  195 

f-  4,  then  the  full  correction  is    +3,  and  the  glasses 

iven  would  be    +2.     With  an  intelligent  quiet  child 

rpectacles  may  be  ordered  at  three  years  of  age,  with 

instructions,  of  course,  for  their  removal  when  the  child 

is  romping. 

If  the  atropine  removes  the  squint,  as  a  rule  the 
glasses  will  also  do  so. 

Be  careful  to  insist  upon  the  constant  use  of  the  glasses, 
and  inform  the  parents  that  this  treatment  will  probably 
have  to  be  persevered  in  for  years. 

If  the  squinting  eye  is  amblyopic,  the  child  should 
be  made  to  practise  it  by  placing  an  opaque  clip  on  the 
glass  of  the  good  eye,  or,  what  is  better  still,  especially 
with  the  very  young,  by  bandaging  up  the  good  eye 
and  forcing  the  bad  eye  to  be  used  for  a  short  time 
every  day — for  instance,  at  mealtime.  In  many  cases 
this  treatment  results  in  great  improvement  of  vision 
(see  page  96) . 

Another  useful  method  of  forcing  the  amblyopic  eye 
to  work,  is  to  instil  atropine  into  the  good,  or  fixing, 
eye  only ;  as  the  accommodation  of  this  eye  is  paralyzed, 
it  can  only  be  used  for  distant  vision,  and  for  near  vision 
the  amblyopic  eye  must  be  used,  and  is  then  forced  to 
work  for  a  considerable  period  of  the  day.  Some  sur- 
geons consider  that  this  treatment  takes  the  place  of,  and 
is  better  than,  excluding  the  fixing  eye  with  a  bandage. 
The  use  of  atropine  in  both  eyes  (except  for  the  pur- 
pose of  estimating  the  error  of  refraction),  which  is 
sometimes  advocated  when  the  child  is  considered  to  be 
too  young  for  glasses,  is  to  be  strongly  condemned. 

As  an  essential  cause  of  convergent  squint  may  be  a 
defective  development  of  the  fusion  faculty,  the  or- 
thoptic training  is  very  important,  and  cannot  be  begun 
too  early. 

Worth  has  devised  an  instrument  called  the  amblyo- 
scope,  which  is  on  the  principle  of  the  stereoscope :  each 
eye  looks  through  a  separate  tube,  and  a  different  part 


196  THE  REFRACTION   OF  THE   EYE 

of  the  whole  picture  is  placed  before  each  tube,  and  so 
arranged  that  when  both  eyes  are  being  used  these  two 
portions  are  fused  into  one  picture.  These  tubes  are 
not  fixed  together,  but  can  be  separated  to  any  angle, 
and  thus  adjusted  to  the  particular  squint  (see  Trans. 
Ophth.  Sqc,  vol.  xxi.,  page  245).  The  picture  before 
each  tube  can  be  separately  illuminated,  and  so,  by 
lowering  the  illumination  of  the  picture  before  the 
better  eye,  or  increasing  that  before  the  worse  eye, 
binocular  fusion  vision  may  be  easily  obtained  in  very 
bad  cases,  and  consequently  Worth's  amblyoscope  is 
far  superior  to  the  numerous  cheaper  stereoscopic 
instruments.* 

The  above  treatment  should  be  given  a  good  trial  as 
long  as  the  smallest  improvement  is  shown;  but  when 
no  improvement  is  taking  place,  and  especially  when  the 
squinting  eye  is  becoming  amblyopic  or  more  amblyopic, 
we  must  resort  to  operation. 

Squint  operations  are  always  best  done  under  cocaine 
and  adrenalin,  because,  when  the  patient  is  conscious, 
we  are  able  to  judge  accurately  the  amount  of  adjustment 
necessary ;  but  when  we  have  to  operate  on  quite  young 
children,  a  general  anaesthetic  is  necessary. 

In  slight  convergent  strabismus,  division  of  the 
internal  rectus  may  be  sufficient;  if  not,  we  must  ad- 
vance the  external  rectus  of  the  same  eye;  and  lastly, 
in  rare  cases,  division  of  the  other  internal  rectus  may 
be  necessary. 

The  younger  the  patient,  the  more  likely  is  the  treat- 
ment of  atropine  (in  the  fixing  eye)  and  glasses  to  succeed 
and  the  operation  to  be  unnecessary ;  but  in  older  sub- 
jects, squints  are  rarely  cured  except  by  tenotomy  or 
advancement,  or  both. 

While   the   spectacle   treatment   is   being   tried,   the 

*  Worth  considers  that  the  training  of  the  fusion  sense  cannot 
be  begun  too  early,  and  that  it  is  rarely  of  any  use  after  the  age 
of  6.  The  surgeon  should  himself  give  the  child  the  "  lesson" 
with  the  amblyoscope  once  a  week  for  at  least  six  weeks. 


STRABISMUS  197 

refraction  should  be  carefully  estimated  under  atropine 
at  least  once  a  year. 

Divergent  Strabismus. — This  condition  is  rarely  seen 
in  young  children,  and,  as  a  rule,  develops  at  puberty  or 
later — in  fact,  when  myopia  is  progressing.  By  stop- 
ping undue  convergence,  and  so  helping  to  restore  the 
fatigued  internal  recti,  we  may  cure  slight  cases  at  the 
onset  by  putting  the  patient  into  glasses;  but  it  is 
very  uncertain,  and  in  the  majority  of  cases,  when  once 
the  "  insufficiency  "  has  passed  into  a  squint,  nothing 
short  of  an  operation  is  any  good.  This  consists 
in  dividing  the  external  rectus,  and,  when  this  is  not 
sufficient,  in  advancing  the  internal  rectus.  Training 
the  eye,  and  so  trying  to  reduce  the  amblyopia,  should 
also  be  resorted  to.  When  the  squint  is  not  very 
marked,  give  the  glasses  and  the  orthoptic  training 
a  fair  trial  of,  say,  six  months  before  resorting  to 
operation. 

After  squint  operations  it  is  most  important  for  the 
patient  to  continue  to  wear  the  correction,  and  also  to 
persevere  with  the  stereoscopic  training.  He  should  be 
warned  that,  unless  this  is  done,  there  is  a  danger  of 
further  trouble — for  instance,  a  convergent  strabismus 
that  has  been  corrected  may  develop  into  a  divergent 
strabismus,  etc. 

In  oldish  patients  with  an  old  convergent  squint, 
where  the  squinting  eye  is  not  amblyopic,  and  when  an 
operation  is  refused  or  deemed  inadvisable,  if  diplopia 
is  present,  prisms  base  out  must  be  ordered  with  the 
correction. 


CHAPTER  XV 

CYCLOPLEGIA,  CYCLOPLEGICS,  AND  CILIARY  SPASM 

Cycloplegia. — Cycloplegia,    or   paralysis  of   the   ciliary 
muscle,  may  be  due  to — 

1.  Drugs,  such  as  atropine. 

2.  Systemic      poisons  —  diphtheria,      influenza, 

syphilis,  etc. 

3.  Disease  of  the  nervous  system,  concussion  of 

the  brain,  etc. 

Cycloplegics. — A  cycloplegic  is  a  drug  which  tem- 
porarily paralyzes  the  ciliary  muscle,  and  by  its  use  we 
are  enabled  to  estimate  the  refraction  of  the  eye  at  rest. 
Cycloplegics  are  also  mydriatics — that  is,  they  paralyze 
temporarily  the  sphincter  iridis,  and  cause  dilatation  of 
the  pupil. 

The  only  cycloplegics  we  need  concern  ourselves  with 
in  refraction  work  are  atropine  and  its  derivative,  horn- 
atropine.  These  drugs  paralyze  the  sphincter  iridis  and 
the  oculo-motor  nerve-endings  in  the  ciliary  muscle; 
consequently  the  pupil  is  dilated  and  accommodation 
power  is  reduced,  or  (when  the  full  action  of  the  drug  is 
obtained)  lost,  leaving  the  eye  adjusted  for  its  far  point 
and  in  a  state  of  rest. 

Atropine  is  the  stronger  drug,  and  should  be  used, 
when  practicable,  in  all  young  subjects  where  the  ampli- 
tude of  accommodation  is  very  great.  In  children  under 
16  years  the  full  effect  is  obtained  only  after  two  days' 
use;  in  older  patients  complete  cycloplegia  may  occur 

in  a  few  hours. 

198 


CYCLOPLEGics  tgg 

The  effect  of  the  drug  does  not  begin  to  pass  off  for 
thirty-six  hours,  and  the  accommodation  power  is  not 
fully  restored  until  a  week  or  ten  days  have  elapsed. 
The  pupil  is  restored  to  its  normal  size  about  the  same 
time,  sometimes  a  little  earlier. 

Homatroplne  has  the  same  general  effect  as  atropine, 
but  differs  in  that  its  full  effect  on  the  pupil  and  ciliary 
muscle  manifests  itself  more  promptly,  and  disappears 
much  more  rapidly,  than  atropine ;  but  it  is  not  so  com- 
plete a  paralyzer  of  the  ciliary  muscle  as  atropine,  and  in 
young  people  whose  accommodation  is  very  active  it  is 
not  to  be  relied  on.  On  the  other  hand,  in  most  people 
over  25  years  of  age  it  paralyzes  the  muscles  quite 
enough  for  all  practical  purposes  when  used  in  suffi- 
ciently strong  doses  and  combined  with  cocaine. 

Cocaine  favours  the  absorption  of  the  drug  by  render- 
ng  the  outer  epithelial  layer  of  the  cornea  and  the 
conjunctiva  more  pervious. 

As  a  general  rule,  the  full  effect  of  homatropine  is 
obtained  in  an  hour.  This  effect  begins  to  pass  off  in  two 
hours,  and  the  whole  effect  has  disappeared  in  twenty- 
four  or  twenty-six  hours.  The  restoration  of  the  pupil 
to  its  normal  size  takes  a  few  hours  longer. 

Between  the  ages  of  16  and  20  the  selection  of  the 
cycloplegic  must  depend  on  the  time  that  the  patient  can 
give  up  to  the  examination.  Always  try  and  obtain  con- 
sent for  the  use  of  atropine  in  such  cases,  as  its  effect  is 
more  rehable;  but  where  only  one  day  can  be  spared, 
the  surgeon  will  have  to  be  content  with  homatropine. 
In  young  subjects  at  school  who  cannot  give  up  the 
time  to  atropine  cycloplegia,  homatropine  should  be 
exhibited  two  or  three  times  at  intervals  of  half  an  hour 
before  the  refraction  is  estimated;  but  if  a  satisfactory 
result  is  not  obtained,  atropine  will  have  to  be  used. 
One  great  advantage  of  homatropine  is  that  it  rarely,  if 
ever,  produces  toxic  symptoms. 

The  Form  in  which  Cycloplegics  should  be  used.— It 


200  THE  REFRACTION   OF  THE   EYE 

is  impossible  to  know  when  using  drops  or  solutions 
how  much  of  the  drug  is  absorbed  and  how  much  is 
wasted,  and  they  do  not  keep  well.  Atropine  in  solution 
is  liable  to  produce  toxic  symptoms  by  passing  down  the 
tear  passages  into  the  throat. 

On  the  other  hand,  ophthalmic  "  tabloids  "  and  discs 
have  been  brought  to  such  a  state  of  perfection  that 
they  form  the  most  scientific,  efficient,  and  safe  method 
of  administering  the  drugs. 

The  most  useful  tabloids  are:  atropine  -^l^  gr.,  and 
homatropine  with  cocaine  Jq  gr-  each.* 

Tabloids  or  discs  should  dissolve  quickly  when  placed 
on  the  inner  surface  of  the  lower  lid,  and  should  cause 
little  or  no  irritation  or  pain. 

Atropine  may  also  be  used  in  the  form  of  an  ointment, 
the  pure  alkaloid  being  dissolved  in  vaseline,  the  propor- 
tion being  gr,  iv.  of  atropine  to  the  ounce  of  vaseline. 
This  should  be  put  up  in  a  small  tube,  and  a  small 
quantity  placed  on  the  inside  of  the  lower  lid,  by  means 
of  a  clean  glass  rod,  morning  and  late  afternoon.  It 
should  not  be  used  on  the  day  the  examination  is  to  be 
made,  as  the  presence  of  the  vaseline  may  interfere  with 
the  tests. 

Cycloplegics  are  rarely  necessary  over  the  age  of  45. 
The  accommodative  power  is  considerably  reduced  by 
that  time,  and  any  latent  hyperopia  that  may  have  been 
present  has  become  manifest  (see  Presbyopia,  page  149). 
Never  use  a  cycloplegic  should  there  be  any  suspicion 
of  glaucoma  or  a  tendency  to  glaucoma. 

Make  it  a  rule  never  to  use  atropine  in  patients  with 
high  hyperopia  over  25  years  of  age,  and  then  there 
need  be  little  fear  of  inducing  a  glaucomatous  attack, 
because  homatropine  is  very  speedily  and  efficiently 
counteracted  by  eserine,  and  if  homatropine  has  been 
used,  and  any  suspicious  symptoms  arise,  a  tabloid  of 
eserine  (^^  gr.)  will  allay  all  anxiety. 

*  Burroughs,  Wellcome  and  Co.,  tabloids  "  B  "  and  "  W." 


CILIARY  SPASM  201 

Cycloplegia  following  Diphtheria. — When  the  accommodation 
is  paralyzed  after  diphtheria,  it  generally  occurs  in  young  sub- 
jects, is  bilateral,  and  may  follow  a  most  insignificant  attack  of 
the  disease.  The  patient  is  in  the  same  condition  as  an  old 
person  who  has  lost  all  accommodation,  and  the  treatment  is 
practically  the  same.  If  emmetropic,  reading  glasses  only  are 
necessary,  and  the  weakest  convex  glasses  with  which  reading  is 
possible  should  be  prescribed  in  order  to  encourage  the  ciliary 
muscle  to  act,  and  these  glasses  should  be  changed  for  still  weaker 
ones  as  the  power  returns  to  the  muscle.  When  an  error  of 
refraction  is  present,  bi-focal  glasses  for  distance  and  near  vision 
must  be  ordered  (see  Presbyopia,  page  150). 

As  dilated  pupils  from  iridoplegia  very  often  coexist  with 
cycloplegia,  considerable  improvement  in  vision  may  result  from 
the  use  of  a  drop  of  eserine  in  the  eyes  every  morning. 

When  cycloplegia  results  from  any  other  cause,  the  aforesaid 
treatment  should  be  adopted,  combined  with  the  internal  adminis- 
tration of  iodide  of  potassium  or  strychnine,  etc.,  as  may  be 
indicated. 

Spasm  or  Cramp  of  the  Ciliary  Muscle. — ^This  is  the 
opposite  of  cycloplegia,  and  occurs  in  two  forms:  (i)  A 
temporary  spasm,  soon  passing  off  with  rest;  (2)  a 
permanent  spasm,  referred  to  in  the  previous  pages  as 
spasm  of  accommodation,  and  generally  associated  with 
hyperopia  in  young  people  (see  page  94),  and  producing 
an  apparent  myopia.  Both  forms  are  the  result  of 
strain  of  the  ciliary  muscle,  and  are,  with  rare  excep- 
tions, cured  by  the  use  of  a  cycloplegic  and  the  correc- 
tion pi  the  refraction  error. 

There  is  a  form  of  spasm  of  the  accommodation  which  Leslie 
Paton  calls  "  functional  spasm."  In  the  case  he  cites  {Trans. 
Ophth.  Sac,  vol.  xxxvii.,  p.  370)  a  lady  with  a  small  amount  of 
myopic  astigmatism  at  times  acquired  a  spasm  of  accommodation 
of  9  or  10  D,  accompaniedby"  cramp  of  convergence."  This  con- 
dition is  seen  in  neurasthenic  patients  and  is  produced  by  eye- 
strain. De  Schweinitz  ("Diseases  of  the  Eye,"  8th  edition,  p.  122) 
says:  "  Spasm  is  prone  to  occur  in  individuals  of  neurasthenic 
condition,  and  is  a  frequent  symptom  of  hysteria,  often  associated 
with  cramp  of  convergence." 

This  form  of  spasm  may  occur  in  quite  old  patients,  even  up 
to  45.  Most  meticulous  care  should  be  employed  in  ascertaining 
the  refractive  error,  and  the  proper  correction  will  in  time  effect 
a  cure. 

All  these  forms  of  spasm  are  exaggerations  of  the  tonic  spasm 
that  exists  in  normal  conditions  and  which  disappears  under  the 
action  of  a  cycloplegic. 


CHAPTER  XVI 

METHODS    OF    EXAMINATION— NOTE-TAKING 

Methods  of  Examination. — The  room  should,  if  possible, 
be  sufficientty  long  tv  allow  the  patient  to  be  seated 
6  metres,  or  20  feet,  from  the  type.  When  this  length 
is  not  obtainable  (even  diagonally),  reversed  types  must 
be  used  and  hung  over  the  patient's  head  behind  him, 
and  opposite  should  be  a  mirror,  on  which  the  type  is 
reflected.  The  distance  should  be  so  arranged  that 
the  distance  between  the  patient  and  the  mirror  and 
between  the  mirror  and  the  type  together  measure 
6  metres. 

Apparatus  Required. — The  distant  type  should  be 
Snellen's  type,  and  several  boards,  with  a  different 
arrangement  of  letters,  should  be  used,  and  changed  as 
necessity  arises,  or  they  may  be  arranged  in  a  box  form 
and  rotated  by  a  cord  by  the  surgeon  from  where  he  is 
standing.  Dixey's  test  types*  (Fig.  88)  are  arranged  so 
that  only  one  line  of  type  is  displayed  at  a  time,  the 
different  lines  being  turned  into  position  as  required 
by  a  cord,  as  in  the  box  form  of  type.  The  change  of 
type  prevents  the  patient  from  learning  the  arrange- 
ment of  the  letters. 

The  type  must  be  well  and  evenly  illuminated,  prefer- 
ably by  artifical  light,  and,  if  possible,  in  a  dark  part 
of  the  room,  so  that  the  difference  between  a  bright  and 
dark  day  has  little  or  no  effect  on  the  record. 

*  Dixey,  3,  New  Bond  Street,  W. 
202 


APPARATUS 


203 


A  dark  room,  although  desirable,  is  not  absolutely 
necessary;  the  whole  consulting- room  can  be  darkened 
with  a  blind  or  curtain,  or  a  dark  corner  can  be  curtained 
off.    Absolute  darkness  is  not  a  sine  qua  non. 

The  lighting  should,  if  possible,  be  electric,  and 
ground-glass  lamps  should  be  used,  or  the  special  high 
candle-power  lamp  made  for  eye  or  throat  work,  which 
is  mostly  ground  glass  with  a  small  portion  clear. 
Failing  the  electric  light,  the  "  incandescent "  is, 
perhaps,  the  best  form  of  gas  illumination. 


The  reading  type  should  be  kept  clean  in  a  cover. 
Both  forms  of  type  are  figured  at  the  end  of  the  book. 

The  Trial  Case. — This  contains  pairs  of  concave  and 
convex  spherical  and  cylindrical  lenses  and  prisms. 
The  spherical  lenses  should  be  numbered  in  intervals  of 
•12  to  I,  '25  to  4,  '5  from  4  to  8,  and  in  intervals  of 
I  from  8  to  20. 

The  cylindrical  lenses  should  be  numbered  in  intervals 
of  •12  to  I,  '25  from  i  to  3,  *$  from  3  to  6,  and  intervals 
of  I  from  6  to  8. 


204  THE  REFRACTION  OF  THE  EYE 

The  prisms  should  be  from  i°  to  12°,  or  14°. 

The  lenses  should  be  as  thin  as  possible,  and  they 
should  all  be  mounted  in  thin  frames,  with  a  small  flat 
handle  on  which  the  number  is  engraved. 

When  the  surgeon  does  not  wish  to  start  with  so 
expensive  a  set  of  lenses,  he  can  manage  very  well  with 
a  set  consisting  of  the  following  glasses : 

Sphericals,  convex  and  concave :  30  pairs  each  from 

•12  to  20. 
Cylindricals,  convex  and  concave:   18  pairs  each 

from  •12  to  6. 
Prisms  from  1°  to  12°. 

This  set  in  a  suitable  case  with  stenopaic  discs, 
adjustable  trial  frame,  etc.,  can  be  supplied  by  Messrs. 
Hamblin,  5,  Wigmore  Street,  at  a  reasonable  cost. 

Trial  Frame. — Get  a  really  good  trial  frame,  regard- 
less of  cost.  One  of  the  best  is  made  by  Curry  and 
Paxton.  It  should  be  light  (made  of  aluminium), 
capable  of  being  adjusted  to  fit  any  patient,  and  of  being 
correctly  centred.  It  should  have  a  screw  for  rotating 
the  cylinder  (which  can,  if  necessary,  be  used  by  the 
patient),  for  by  this  means  we  insure  much  greater 
accuracy  in  obtaining  the  correct  angle  of  the  cylinder. 
It  should  be  so  arranged  that  where  two  lenses  are 
used — i.e.,  a  sphere  and  a  cylinder — they  should  almost 
touch,  and  the  back  glass  should  be  as  close  to  the  eye 
as  the  lashes  will  permit.  There  are  many  bad  trial 
frames  on  the  market,  the  worst  example  being  the 
rigid  one  supplied  in  most  trial  cases,  which  is  practically 
useless. 

Besides  the  above  contents  of  the  trial  case,  there 
should  be  several  "  blanks  "  to  block  off  vision,  a  steno- 
paic slit,  a  pin-hole  disc,  and  neutral  tinted  glasses  of 
various  shades. 

Refraction  Ophthalmoscope. — It  is  false  economy  to  buy 
a  cheap  one.  Get  a  good  instrument  to  start  with,  and 
it  will  last  a  lifetime  (see  page  61). 


EXAMINATION   OF   PATIENT 


205 


Ophthalmoscopic  Mirrors. — A  plane  and  a  concave 
mirror  are  wanted,  and  these  may  be  procured  in  one, 
each  mirror  serving  as  the  cover  of  the  other  (see  page  83) . 

Focus-glass. — A  large  focus-glass,  as  described  on 
page  62. 

Maddox  Apparatus. — The  test-board,  near  test-type, 
rod  and  prism  (see  page  45). 

Perimeter. — McHardy's  recording  perimeter  is  the 
best,  but  is  expensive;  there  are  many  almost  as  useful, 
and  much  cheaper. 

The  Ophthalmometer  is  an  expensive  instrument,  but 
is  really  indispensable  (see  page  126). 

Cycloplegics. — Tabloids  of  homatropine  and  cocaine, 
aa  gr.  -gV;  atropine,  gr.  ojxri  and  eserine,  gr.  u^. 


Fig.  89. 


Prescription  forms  for  spectacles  are  supplied  by  most 
opticians,  or  they  can  be  engraved  or  stamped  on  the 
surgeon's  own  paper,  as  illustrated  in  Fig.  89. 

Mark  the  axis  by  writing  the  degree  as  indicated  at 
page  138. 

Note-Making. — It  is  very  useful  in  recording  notes  of 
patients  to  have  some  scheme.  Fig.  90  is  a  suggested 
form  when  using  the  card  system,  but  if  the  surgeon 
wishes  more  voluminous  notes,  and  especially  if  he 
prefers  a  bound  book,  the  ophthalmic  case-book,  pub- 
lished by  Pulman,  of  Thayer  Street,  W.,  will  meet  all 
his  wants. 

The  Systematic  Examination  of  the  Patient. — After 
recording  name,  age,  history,  and  symptoms,  the  patient 
is  placed  in  the  chair  opposite  the  type,  and  the  trial 


206 


THE  REFRACTION  OF  THE   EYE 


EXAMINATION   OF  PATIENT  207 

frame  is  adjusted.  With  one  eye  blocked,  the  visual 
acuity  of  each  eye  is  separately  recorded,  and  also 
roughly  the  effect  of  concave  or  convex  glasses.  There 
is  no  necessity  to  waste  much  time  over  the  first  examina- 
tion if  a  cycloplegic  is  going  to  be  used.  On  the  patient's 
return  under  atropine  or  homatropine,  take  him  straight 
into  the  dark  room,  examine  carefully  with  the  ophthal- 
moscope, and  then  ascertain  the  refraction  by  retinos- 
copy.  If  you  have  an  ophthalmometer,  measure  the 
astigmatism.  Then  take  the  patient  back  to  the  type 
examination,  find  out  the  glass  or  combination  of  glasses 
that  give  best  vision,  record  this,  and  let  him  return  for 
a  final  visit  when  the  effect  of  the  cycloplegic  has  passed 
off,  when  you  order  the  correction.  When  the  patient 
cannot  return  for  a  third  visit,  you  must  order  the 
glasses  according  to  the  rules  laid  down  in  the  previous 
chapters. 

Of  course,  when  a  cycloplegic  is  not  used,  the  examina- 
tion is  completed  in  one  visit. 

Remember  that  in  old  patients  hyperopia  is  often 
present  and  absolute,  and  a  weak  convex  glass  will  often 
improve  vision  from  /^  to  i. 

Always  note  and  record  the  spectacles  that  have  been 
previously  worn. 


CHAPTER   XVII 

SPECTACLES 

Spectacles. — The  treatment  of  errors  of  refraction  can- 
not be  considered  to  be  complete  unless  the  optician 
has  accurately  made  up  the  prescription,  consequently 
it  is  very  important  for  the  surgeon  to  check  the  glasses 
and  their  fit ;  if  practicable,  this  should  never  be  omitted. 
The  optical  centre  of  the  glass  should  coincide  with  the 
visual  axis.  Distance  glasses  should  be  centred  for 
distance,  and  near-work  glasses  for  the  point  at  which 
they  are  intended  to  be  used.  Glasses  for  constant  use 
should  be  centred  for  a  point  between  these  two. 

Glasses  have  a  prismatic  effect  if  decent  red.  A  con- 
vex glass  may  be  said  to  consist  of  two  prisms  with  the 
bases  in  contact.  If  the  glasses  are  too  wide  apart, 
the  patient  looks  through  the  inner  side  of  the  glass, 
which  has  the  same  effect  as  looking  through  a  prism 
with  its  base  outwards  (see  Fig.  78) ;  consequently  the 
convergence  effort  will  have  to  be  increased.  If  the 
glasses  are  too  close  together,  we  have  the  same  effect 
as  a  prism  with  its  base  inwards,  and  the  convergence 
effort  will  be  diminished,  the  accommodation  being  in 
excess.  Concave  glasses  may  be  said  to  be  two  prisms 
with  their  apices  in  contact,  and  the  effect  of  their  being 
out  of  the  centre  is  the  reverse  of  that  of  convex  lenses 
(see  Fig.  77).  When  these  results  are  not  desired — 
that  is,  when  the  lenses  are  not  purposely  decentred — 
it  is  easy  to  understand  how  badly-fitting  spectacles 
may  be  worse  than  useless. 

208 


SPECTACLES 


20^ 


To  check  the  centring  of  the  glasses,  draw  a  thick 
vertical  ink  line  on  a  piece  of  paper,  hold  the  lens  at  a 
slight  distance  from  it,  and  move  it  from  side  to  side; 
when  the  lens  is  convex,  the  line  seen  through  the  lens 
will  appear  to  move  in  the  opposite  direction ;  when  con- 
cave, in  the  same  direction.  In  Fig.  91  a  convex  glass 
has  been  moved  to  the  right,  and  the  line  seen  through 
the  lens  has  moved  to  the  left.  By  moving  the  lens 
from  side  to  side  the  neutral  part  will  be  found  where  the 
lines  are  continuous ;  this  is  the  optical  centre  for  lateral 
movement.    Mark  this  point  as  a  short  vertical  line  on 


Fig.  91. 


the  lens  with  ink  or  a  special  grease  pencil;  now  turn 
the  lens  round  through  90^  and  proceed  as  before; 
where  the  two  lines  meet  is  the  optical  centre  of  the  lens, 
and  this  should  correspond  with  the  centre  of  the  pupil 
when  the  eyes  are  adjusted  for  the  distance  for  which 
the  glasses  are  ordered. 

If  a  prismatic  effect  is  desired  and  ordered,  then  the  op- 
tical centre  should  be  decent  red  as  explained  on  page  178. 

The  plane  of  the  glasses  should  be  perpendicular  to 
the  visual  axis  when  in  use,  hence  reading  glasses  should 
be  tilted  forwards. 

The  best  form  of  bridge  is  the  saddle-bridge ;  it  should 
be  flat  in  order  not  to  indent  the  nose,  and  should  fit  the 

14 


210  THE   REFRACTION   OF  THE   EYE 

nose  accurately.  It  is  not  unusual  to  find  that  the 
lower  elbows  (Fig.  92,  b)  are  touching  the  sides  of  the 
nose,  but  that  the  upper  arch  (Fig.  92,  a)  is  not  in 
contact.  If  the  bridge  is  not  well  made,  the  spectacles 
will  slip. 

Sometimes,  however  carefully  the  bridge  is  adjusted, 
it  indents  the  nose  in  young  children,  and  perhaps  tends 
to  interfere  with  the  development  of  the  nose ;  Hamblin's 
"  scholar's  frame  "  remedies  this,  as  the  bridge  is  lifted 
off  the  nose  by  side  clips,  the  glasses  having  the  appear- 
ance of  a  combined  spectacle  and  pince-nez. 

The  glasses  should  be  as  near  the  eyes  as  the  lashes 
will  permit;  13  to  14  mm.  is  the  average  distance;  but 
when  the  lashes  are  very  long,  this  distance  will  have  to 
be  increased.     It  is  most  important  to  remember  that 


Fig.  92. 

the  lashes  must  not  touch  the  glass ;  when  the  lashes  are 
very  long,  periscopic  lenses  should  be  ordered. 

Concave  glasses  are  weakened  and  convex  glasses 
strengthened  by  removing  them  further  from  the  eyes, 
and  vice  versa. 

The  sides  of  the  frame  should  touch  the  temples  and 
pass  behind  the  ears  in  all  spectacles  made  for  children, 
and  preferably  in  every  case  where  the  glasses  are  to  be 
worn  always.  The  portion  behind  the  ear  should  fit 
comfortably,  so  that  the  wearer  is  hardly  conscious 
of  it. 

Under  no  circumstances  should  anyone  but  an  em- 
metropic presbyope  be  allowed  to  wear  folders.  Folders 
never  fit,  are  rarely  correctly  centred,  and  tend  to 
become  bent,  so  that  one  or  .both  glasses,  are  oblique 


SPECTACLES  211 

to  the  plane  of  the  eyes,  and  one  is  often  nearer  the  eye 
than  the  other. 

A  cylindrical  lens  becomes  more  cylindrical,  and  a  spherical 
one  sphero-cylindrical,  by  being  placed  obliquely  before  the 
eyes. 

Pince-nez  should  be  rigid  and  light.  If  they  are  worn 
always,  they  should,  preferably,  be  frameless.  The  clips 
which  keep  pince-nez  in  position  on  the  nose  should  be 
made  of  malleable  material,  so  that  they  can  be  shaped 
to  the  sides  of  the  nose,  and  the  surface  next  the  nose 
should  be  rough.  It  is  most  important  that  all  glasses 
should  "sit"  horizontally;  especially  is  this  the  case 
when  cylinders  are  worn. 

Monocles  may  be  allowed  in  cases  of  monocular 
amblyopia  (see  page  I57)- 

Children  should  always  have  spectacles,  and  not  pince- 
nez,  and  the  frames  ought  to  be  strong  and  light. 

Crookes*S  Glass. — As  far  as  we  know  the  infra-red  rays  are 
of  no  use  to  us  for  visual  purposes,  but,  bearing  in  mind  the  possi- 
bility that  their  continued  action  maybe  harmful  to  the  eye,  it  is 
advisable,  in  selecting  an  anti-glare  glass,  to  choose  one  that  cuts 
off  the  infra-red  rays,  but  not  at  the  expense  of  the  all-important 
screening  off  of  the  ultra-violet  ones.  At  the  same  time,  if  such 
a  glass  has  a  slight  neutral  tint  a  small  amount  of  luminosity 
is  cut  off  and  renders  it  more  useful  against  glare. 

After  an  enormous  number  of  experiments  extending  over  some 
years.  Sir  William  Crookes  has  succeeded  in  finding  such  a  glass, 
or,  rather,  many  such  glasses.  Messrs.  Chance  Bros,  have  put  on 
the  market  two  glasses,  Crookes  "  A  "  and  Crookes  "  B." 

Crookes  "  A  "  is  practically  No.  187,  and  its  composition  is: 

Fused  soda  flux  . .  . .  . .      83-0 

Cerium  nitrate,  crystallised  . .  . .       i7«o 

But  as  it  is  difficult  commercially  to  get  a  pure  cerium  salt,  there 
is  present  a  small  amount  of  didymium,  which  gives  the  glass  a 
pale  pink  tint  when  viewed  edgeways.  This  glass  cuts  off  27  per 
cent,  of  the  heat  rays,  practically  all  the  ultra-violet  rays  (the 
limit  being  \  3650),  and  transmits  99  per  cent,  of  the  light,  and 
is  of  the  greatest  value  in  refraction  work,  especially  in  the 
correction  of  myopia,  and  the  extra  cost  above  ordinary  glass 
is  trifling. 


212  THE  REFRACTION  OF  THE  EYE 

Crookes  "  B  "  has  a  slightly  darker  tint,  and  corresponds  to 
No.  197.     Its  composition  is: 


Fused  soda  flux 
Cerium  nitrate,  crystalUsed 
Nickel  sulphate,  crystallised 
Cobalt  sulphate,  crystallised 
Uranoso-uranic  oxide     < . 

.      79-00 

.       20-50 

0-30 

0*05 

0-I5 

lOO-OO 

It  is  opaque  to  ultra-violet  rays  of  shorter  wave  length  than 
X  3700,  and  cuts  oh  41  per  cent,  of  heat  rays.  It  is  transparent 
to  45  per  cent,  incident  light. 

It  is  not  to  be  recommended  for  general  use,  but  is  especially 
indicated  when  very  great  glare  or  heat  is  encountered. 

It  is  sometimes  very  useful  in  high  myopia. 

Final  Note. — Patients  ought  always  to  be  reminded 
that  the  treatment  of  their  error  of  refraction  is  by  no 
means  permanent.  Changes  will  take  place.  Young 
patients  should  be  re-examined  at  least  once  a  year 
(see  page  98),  older  ones  every  two  or  three  years. 


CHAPTER  XVIII 
ILLUSTRATIVE  CASES 

Simple  Hyperopia  and  Anisometropia. — Miss  L.    aged  21,  com 
plained  of  headaches,  especially  after  doing  any  near  work. 

V.=f+-50  Hm.  B.E. 
Under  atropine  : 

R.V.  A+i-75=f.  L.V.^V+i-50=|. 

No  astigmatism. 

Ordered +'75  sph.+  '50  sph. 
To  be  worn  especially  when  doing  near  work. 

Her  headaches  disappeared. 

Simple  hyperopia  without  astigmatism  or  anisometropia 
rarely  gives  rise  to  symptoms  in  young  people  unless  the  error 
is  great. 

Simple  Myopia— Heterophoria. — Mr.  S.,  aged  25,  clerk.  SufEers 
from  chronic  conjunctivitis,  which  has  been  getting  worse  lately. 
Has  worn  glasses  for  distance,  but  never  for  near  work. 

V.  =  <A-3-5=f  B.E. 
His  accommodation  near  point  is  9  cms. 
,..  a=i^-3-5 
=  11-3-5 
=  7-5 
His  convergence  near  point  is  18  cms.     He  has  i  m.a.  latent 
divergence  for  distance,  and  i'5  m.a.  for  \  metre  (Maddox). 

...ca=:SS^-(-i) 

=  5-5+1 
=  6'5  m.a. 

He  was  ordered  -  3*5  B.E.,  to  be  worn  always,  and  he  was 
specially  instructed  never  to  approach  his  work  nearer  than 
33  cms.  Some  months  later  he  returned,  showing  great  improve- 
ment. There  was  no  heterophoria  for  distance,  and  only  '5  m.a. 
for  J  metre. 

213 


214  THE  REFRACTION  OF  THE  EYE 

Hyperopic  Astigmatism — Epilepsy. — Master  A.  B.,  aged  lo, 
has  had  epileptic  attacks  for  some  years,  with  only  slight  benefit 
from  medical  treatment. 

V.  =1  in  both  eyes. 

Under  atropine  : 

R  E  /  +  '^5  cyl.  (axis  vert.)  \  _  , 
•    '\+ 1  sph.  /     ^' 

T  p  / +-50  cyl.  (axis  vert.)\_g 

The  cylinders  were  ordered  for  constant  use.  A  report,  re- 
ceived eight  months  later,  stated  that  the  boy  had  been  perfectly 
well  since  wearing  the  glasses. 

Hyperopic  Astigmatism  (Low). — Miss  N.,  aged  24,  has  of  late 
been  suffering  intensely  from  headaches,  always  aggravated  by 
near  work, 

V.=^B.E.     NoHm. 

Under  homatropine  : 

V  -«6  /  +  '25  cyl.  (axis  vert.)  \  _  «  ^  p 
^~^\  +  -50sph.  f-s^-^- 

The  ophthalmometer  shows  -25  direct  astigmatism. 

Ordered  4- '25  cyl.  (axis  vert.).     B.E. 

for  constant  use. 

The  wearing  of  this  correction  completely  cured  the  patient. 

Hypermetropic  Astigmatism  (Very  Low) — Anisometropia — Eye- 
strain.—  Private  S.,  suffering  from  shell-shock,  aged  22. 
Great  lassitude;  thoroughly  ill — headache,  insomnia,  no  or- 
ganic lesion. 

V.  =f  in  both  eyes. 

Under  atropine  : 

RV  -  6/  +  -i2cyl.t\6  LV-f  +  '^5cyl.\i  =  |^ 

^•^•-T5|  +  .62sph.    /«•  ^•^•\  +  -50sph.        P- 

The  following  glasses  were  ordered  for  constant  use  : 

R-E-{t:;'s^h.''''""'"''*  L.E,  +  .25cyl.V 

and  in  a  few  weeks  he  was  quite  well. 

Astigmatism— Anisometropia.— Miss  B.,  aged  30,  complains  of 
"  dreadful  headache  "  and  insomnia. 

R.V.  «.  L.V.  f . 

Under  homatropine  the  ophthalmometer  showed: 

R.E.    '5  direct  astigmatism. 
L.E.  I 'S  direct  astigmatism. 


Retinoscopy  : 


ILLUSTRATIVE   CASES  215 

R.E.  L.E. 


+  1' 


+  2-5 


+  1  +1 

K.Y.^+      -5  cyl.  (axisvert.)=t. 
L.V.1%  +  1.25  cyl.  (axis  vert.)  =|. 

The  following  glasses  were  ordered : 

For  constant  use  : 

/  +  -5  cyl.  (axis  vert.) 

J  ^    r+  1-25  cyl.  (axis  vert.) 

Note. — On  returning  for  the  final  examination  when  the  effects 
of  the  cycloplegic  had  passed  off,  it  was  found  that  the  patient  pre- 
ferred '75  off  the  left  and  only  -50  off  the  right.  When  possible, 
it  is  always  best  to  take  a  little  more  off  the  stronger  glass,  with 
the  view  of  lessening  the  difference  in  the  correction  of  the  two 
eyes,  at  least  in  one  meridian. 

Simple  Myopic  Astigmatism.— Mr.  P.,  aged  29.  Has  had 
migraine  for  some  time. 

V.=f  B.E. 

Under  homatropine  :  ophthalmometer  shows  -5  direct  astig- 
matism, and  vision  =1  c  -  '5  cyl.  (axis  horizontal)  B.E.  On  re- 
covering from  the  cycloplegic,  -  '25  sph.  added  to  the  cyl. 
improved  each  eye  separately,  but  binocularly  the  cyl.  only  gave 
him  f ,  and  these  glasses  were  ordered  for  constant  use,  and  some 
weeks  afterwards  the  patient  reported  that  the  migraine  had 
disappeared. 

Myopic  Astigmatism  (Very  Low).— Miss  N.  B.,  aged  30,  has 
suffered  for  some  years  from  "  nerves,"  and  is  now  complaining 
of  dyspepsia.  Vision  in  both  eyes  was  §  part,  and  the  ophthal- 
mometer showed  inverse  astigmatism  '12  in  both  eyes. 

Under  homatropine  : 

,  f-'i2  cyl.  (axis  vert.)\  _    ^  ^ 
r^\+-50sph.  ^—^o.tL. 

The  minus  cylinder  was  given  her  to  wear  always,  and  sbi 
months  later  she  reported  that  she  was  perfectly  well  and  the 
dyspepsia  had  entirely  disappeared. 

Myopic  Astigmatism  (Compound). — Miss  B.,  aged  28.  Ha 
uever  worn  glasses.     Complains  of  dreadful  headaches. 

V.^-^;^"^  and -4=^  B.E, 


2l6  THE  REFRACTION  OF  THE   EYE 

Under  homatropine  the  ophthalmometer  showed : 
R.E,  I  inverse  astigmatism. 
L.E.  '5  inverse  astigmatism. 

R  V  /  ~  ^  ^y^-  ^^^^  vert.)-!  _„ 
•     X-asph.  f-^ 

L  V  /  -  -5  cyl.  (axis  vert.) \  _  ^ 
•'•\-3.5sph.  /-«• 

These  glasses  were  ordered  for  constant  use,  and  the  patient 
reported  that  the  headaches  disappeared. 

Note. — The  above  is  also  an  example  of  anisometropia,  and  it  is 
very  commonly  found  in  those  cases,  as  above,  where  the  differ- 
ence is  not  great,  that  in  one  axis  the  refraction  is  the  same  in 
both  eyes.     Here  the  horizontal  meridian  is  -  4  in  both  eyes. 

Mixed  Astigmatism. — Miss  H.,  aged  23,  complains  that  her 
vision  is  very  bad,  and  that  the  eyes  are  very  painful.  Never 
worn  glasses. 

V.  =-^  in  both  eyes. 

Under  homatropine  the  ophthalmometer  showed  2*5  astigma- 
tism oblique  in  both  eyes. 

Retinoscopy  : 

X        X 

4-2       --5  -1+2 

45 

^•^•l-i-5sph.     /-Tir- 
40 

^•^•\-2sph.     /-r^- 

These  glasses  were  ordered  for  constant  use. 

In  cases  of  which  above  is  an  example  visual  acuity  is  gene- 
rally poor,  but  great  improvement  results  a  few  months  after 
wearing  the  correction. 

Mixed  Astigmatism  (Low)  and  Anisometropia  and  Eyestrain. — 
Miss  T.  P.,  aged  18,  had  had  headaches  and  spinal  pain  for  four 
years. 

R.  andL.V.=f. 

Under  atropine  : 

The  following  glasses  were  ordered  for  constant  use: 

^  ^  /  +  -37  cyl-  (axis  vert.).       j  ^  /  +  -25  cyl.  (axis  vert.). 
■^"    •\  - -12  sph.  •^*\-«i2sph, 


ILLUSTRATIVE  CASES  217 

She  immediately  began  to  improve,  and  when  last  heard  of  was 
feeling  better  every  week. 

Simple  Presbyopia. — Mr.  H.,  aged  53.  He  says  his  distant 
vision  is  good,  but  that  the  glasses  he  uses  for  near  vision  (+1) 
do  not  give  him  the  help  they  did,  and  after  reading  a  short  time 
the  print  becomes  confused,  and  he  has  to  rest  a  short  time  before 
resuming  reading. 

V.=f.     No  Hm.  B.E. 

He  reads  D=0'5  well  c.  +  2  sph.,  and  the  ophthalmometer 
shows  no  astigmatism.      +2  were  ordered  for  all  near  work. 

Simple  Hyperopia  and  Presbyopia. — Mr.  E.,  aged  58.  Has  a 
considerable  amount  of  conjunctivitis,  from  which  he  has  been 
suffering  for  a  year.     Is  using  -f  2'$  sph.  B.E. 

R.V.^«^+ 1.25=1. 
L.V.^+1.5   =t. 

The  ophthalmometer  shows  no  astigmatism.  Reads  D=0'5 
well  c  +  4  sph. 

He  refused  glasses  for  distance,  and  was  ordered  +  4  sph.  for 
all  near  work. 

Note. — He  preferred  the  same  glass  in  both  eyes. 


Simple  Myopia  and  Presbyopia. — Mr.  O.,  aged  52. 
R.V. 
L.V. 


;}=A-3=f. 


The  ophthalmometer  shows  no  astigmatism.     Reads  D=0'5 
well  c  -  i'5  sph. 

He  was  ordered  the  following  glasses: 

The  patient's  muscle  test  was  normal,  and  he  had  no  symptom 
of  eyestrain. 

Hyperopic  Astigmatism  and  Presbyopia. — Miss  E.,  aged   52, 
complains  of  constant  headache. 

R  V  _  «/  +  '75cyl.  (axis  vert.)  "i_ 
^•^•-^*\+i.5  sph.  /"*• 

The  ophthalmometer  shows  astigmatism  as  under: 

R.E.  '75  direct. 

L.E.  .50  direct  and  oblique. 

With  +  2'5  added  to  above  she  read  D  =0'5  well. 
Bi-focal  glasses  Qrd^red  fgr  constant  use,  with  correction  as 
ftbpve, 


2X8  THE   REFRACTION   OF   THE   EYE 

Some  months  later  patient  reported  that  the  headaches  had 
entirely  disappeared . 

Note. — It  will  be  noticed  that  the  ophthalmometer  gave  in  the 
left  eye  a  lower  correction  than  that  which  the  patient  preferred. 
This  constantly  occurs  in  oblique  astigmatism,  and  it  is  probably 
due  to  the  presence  of  a  slight  amount  of  static  lenticular  astigma- 
tism, which,  of  course,  the  ophthalmometer  cannot  recognize. 

Myopic  Astigmatism  and  Presbyopia. — Mr.  B.,  aged  46,  com- 
plains of  twitching  of  the  eyelids  and  a  strained  feeling  about 
the  eyes  after  reading, 

anil-  '75  cyh  (axis  vert.)  j  _, 
L.V.J-*75sph.  /      " 

The  ophthalmometer  showed   inverse  astigmatism   -75,     He 
reads  easily  D=0'5  with  -1- '75  cyl,  (axis  horizontal). 
The  following  glasses  were  ordered : 

Distance:  r  _  .75  cyl.  (axis  vert.) 

^•^•\--75sph. 
Reading  :  ^^  ^  ^^^  ^^^   ^^^.^  j^q^.^^.)  . 

He  preferred  to  have  two  pairs  of  glasses,  and  not  bi-focals. 

Spasm  of  Accommodation.— Master  L.,  aged  14.  Has  had 
some  trouble  with  the  eyes  for  some  time,  with  headache  after 
near  work.  He  saw  an  optician,  who  tested  his  vision  (without 
atropine),  and  the  boy  chose   -  3  sph.,  which  were  ordered  him. 

Under  atropine  : 

T^  V  «  /  +  '25  cyl.  (axis  horiz.)  \  _  „ 

^^^"^  I +  -50  sph.  j  -^'• 

T  \T   c  /  + '12  cyl.  (axis  horiz.) \  _g 

•   •^M  + -50  sph,  /-o- 

The  ophthalmometer  showed  -25  inverse  astigmatism  for  the 
R.E,,  and  '12  for  left.  The  cylinders  were  ordered  for  constant 
use,  and  in  a  few  weeks  the  spasm  entirely  disappeared. 

Note. — This  case  shows  the  danger  of  giving  glasses,  especially 
concave  glasses,  to  young  people,  without  using  a  cycloplegic. 
The  eyestrain  had  produced  spasm  of  the  ciliary  muscle,  which 
masked  the  real  condition,  and  made  him  appear  to  be  m5^opic. 

High  Myopia  (with  Astigmatism)    (Progressive  ?)  .—Miss   B„ 

aged  17.     Has  been  wearing  for  some  time  the  following  glasses: 


-  2  cyl.  (axis  horiz.)  1  -□  ^ 
-10  sph.  I^-^- 


Originally  these  helped  her  considerably  in  distant  vision,  but 
now  they  are  only  useful  when  reading,  and  even  when  using  the 
glasses  she  holds  her  book  12  cms.  from  the  eyes. 


\ 


ILLUSTRATIVE   CASES  SIQ 

Under  atropine  :  lo 

RV.{  :  \i^ll  *^}  =  A  one  letter. 

2 

L-V-{:!5Vp''h:'^}=A  one  letter. 

The  ophthalmometer  shows: 

R.E.  I '5  astigmatism,  direct  and  oblique. 
L.E.  2'0  astigmatism,  direct  and  oblique. 

Retinoscopy  : 

R.E.  L.E. 

-17  V^/ 

-19  -17 

The  atropine  correction  was  ordered  for  constant  use,  and  the 
patient  was  strictly  enjoined  never  to  approach  her  work  nearer 
than  33  cms. 

Considering  the  high  myopia,  the  fundus  of  both  eyes  was  fairly 
normal,  although  marked  crescents  were  present.  Three  years 
later  with  the  same  glasses  she  read  the  whole  of  ^^  and  D=0'5 
well  at  33  cms.  The  eyes  were  again  put  under  atropine,  and 
the  refraction  was  found  to  be  practically  the  same  as  at  the 
first  examination. 

Note. — This  case  is  a  marked  example  of  the  importance  of 
preventing  undue  convergence  in  high  myopia,  and  also  illus- 
trates the  admirable  result  of  fully  correcting  the  error. 

Aphakia. — Mrs.  P.,  aged  58.  Right  eye:  Advanced  cataract. 
Projection  good.  Left  eye:  Commencing  cataract;  vision  =  j^. 
Right  cataract  extracted,  and  five  months  later,  after  needhng, 
vision  was : 

+ 13  sph. 


4-  2  cyl, 


^•) 


These  glasses  were  ordered  to  be  used  for  a  short  time  every 
day,  and  later  on,  when  the  cataract  in  the  left  eye  had  advanced, 
+  3  was  added  for  reading  and  the  glasses  ordered  to  be  worn 
as  bi-focals. 

Myopic  Astigmatism — Esophoria. — Master  S.  C,  aged  14. 
Vision  =  f  in  both  eyes.     Under  atropine  vision  was: 

R.V.Apart{;;|3%^y{,-.(---^*)}=t. 

L.V.Apart--75cyl.  •j  =  t. 


220  THE  REFRACTION  OF  THE  EYE 

The  ophthalmometer  showed  '5  inverse  astigmatism  in  the 
right  eye,  and  in  the  left  ^ys  inverse  astigmatism,  slightly  oblique. 
The  Maddox  test  showed  1-5  metre  angles  of  esophoria.  He  had 
been  wearing  4°  base  out  in  both  eyes,  with  no  good  result, 
sympto«is  being  headache  and  general  malaise.  The  foUowmg 
glasses  were  ordered  for  constant  use: 

K.E.  -  '50  cyl,  (axis  vert.) . 
L.E. -.75cyl.l/ 

He  had  been  in  the  habit  of  approaching  his  work  very  near 
to  the  eyes,  and  no  doubt  this  produced  a  spasm  of  the  internal 
recti,  and  the  trouble  had  been  kept  up  by  eyestrain.  He  re- 
covered completely  in  a  few  weeks. 

Muscle  Strain— Convergence  "  Insufficiency  "—Myopia— Aniso- 
metropia.—F.  L.,  aged  31,  a  clerk,  suffering  from  slight  chronic 
conjunctivitis,  complains  of  headache,  chiefly  frontal,  coming  on 
after  work;  says  that  lately  the  headache  appears  before  he  has 
been  two  hours  at  work. 


"H 


L.  <  ^\  -  I   D. 


His  convergence  near  point  is  18  cms,;  if  he  is  told  to  look  at 
the  tip  of  a  pen  at  this  distance,  and  any  attempt  is  made  to  bring 
it  nearer,  the  right  internal  rectus  is  seen  to  suddenly  give  way, 
the  right  eye  turns  considerably  outwards,  and  diplopia  super- 
venes. Examined  by  the  Maddox  test,  he  shows  ^  m.a.,  latent 
divergence  for  distance,  and  i  m.a.,  which  soon  becomes  2  m.a., 
for  I  metre.  He  was  given  a  mixture  of  iron  and  strychnine,  and 
advised  to  take  a  fortnight's  rest,  and  on  returning  at  the  end  of 
that  time  the  following  great  improvement  was  noted:  c  P  — 
8  cms.  Latent  divergence  for  distance  =  ^  m.a.,  and  at  |  metre  = 
}  m.a.  His  range  of  convergence  had  increased  from  6  m.a.  to 
12  m.a.  He  was  advised  to  return  to  business,  and  was  ordered 
the  above  correction  for  constant  use,  and  told  not  to  approach 
his  work  nearer  than  25  cms.  He  eventually  became  completely 
well. 

Convergent  Concomitant  Strabismus. — Master  E.,  aged  8.  Has 
marked  convergent  strabismus,  which  is  alternating,  although  he 
most  frequently  fixes  with  the  left  eye. 

L.V;}=t+i-Hm. 

Under  atropine  for  a  fortnight  the  squint  entirely  disappeared, 
and  +2  =1  B.E.  He  was  given  +  i  sph.  for  both  eyes  for  con- 
stant use  in  circular  frames. 

A  year  later  the  atropine  correction  was  only  i  '5,  and  -t-  '75  was 
ordered;  eighteen  months  later  the  atropine  correction  was  only 
-I- I,  and,  as  the  squint  had  then  entirely  disappeared,  glasses 
were  discontinued. 


CHAPTER  XIX 

VISION  TESTS  FOR  THE  SERVICES 

I  AM  indebted  to  the  heads  of  the  various  departments 
of  the  pubUc  services  for  their  courtesy  in  supplying 
me  with  these  revised  standards. 


COMMISSIONS  IN  THE  REGULAR  ARMY  AND  SPECIAL 
RESERVE.* 

(a)  Squint,  or  any  morbid  condition  of  the  eyes  or  of  the  lids 
of  either  eye  liable  to  the  risk  of  aggravation  or  recurrence, 
will  cause  the  rejection  of  the  candidate. 

(6)  The  examination  for  determining  the  acuteness  of  vision 
includes  two  tests:  one  for  distant,  the  other  for  near, 
vision.  The  Army  test-types  will  be  used  for  the  test  for 
distant  vision,  without  glasses,  except  where  otherwise 
stated  below,  at  a  distance  of  20  feet;  and  Snellen's  Opto- 
typi  for  the  test  for  near  vision,  without  glasses,  at  any 
distance  selected  by  the  candidate.  Each  eye  will  be  ex- 
amined separately,  and  the  lids  must  be  kept  wide  open 
during  the  test.  The  candidate  must  be  able  to  read  the 
tests  without  hesitation  in  ordinary  daylight. 

(c)  The  standards  of  the  minimum  acuteness  of  vision  with 
which  a  candidate  will  be  accepted  are  as  follows : 

Standard  I. 
Right  Eye.  Left  Eye. 

Distant  vision:  V.  =f.  V.  =|. 

Near  vision:  Reads  o,  6.  Reads  o,  6, 

Standard  II. 
Better  Eye.  Worse  Eye. 

Distant  vision:  V.=f.  V.,  without  glasses  =  not  below 

-^^\     and,     after     correction 
with  glasses  =  not  below  ^-^. 
Near  vision:  Reads  o,  6.  Reads  i. 

*  These  regulations  were  received  from  the   War  Office  in 
November,  1917. 

221 


222  THE  REFRACTION   OF  THE  EYE 

Standard  111. 
Better  Eye.  Worse  Eye. 

Distant    vision:     V.,     without  V.,  without  glasses  =  not  belovf 
glasses  =  not  below  ^\;  and,         ^^•,     and,     after    correction, 
after  correction  with  glasses         with  glasses  =  not  below  /^j . 
=  not  below  f. 

Near  vision:  Reads  o,  8.  Reads  i. 

{d)  In  Standard  III.,  the  standard  for  the  test  for  distant  vision, 
without  glasses,  for  officers  of  the  Special  Reserve,  will  be 
not  below  -^. 

{e)  Inability  to  distinguish  the  principal  colours  will  not  be 
regarded  as  a  cause  for  rejection,  but  the  fact  will  be  noted 
in  the  proceedings  and  the  candidate  will  be  informed. 

(/)  The  degree  of  acuteness  of  vision  of  all  candidates  for 
commissions  will  be  entered  in  the  proceedings  in  the 
following  manner : 

(i.)  Candidates  whose  vision    fulfils    the    requirements    of 
Standard  I. : 

"  Vision,    normal." 

(ii.)  Candidates  whose  vision  does    not   fulfil   the  require- 
ments of  Standard  I. : 

V.R.  = ;   with  glasses  = ;   Reads 

V.L.  =  ..... ;   with  glasses  = ;   Reads 

{g)  No  relaxation  of  the  standard  of  vision  will  be  allowed. 


ROYAL  AIR  SERVICE. 

"  Vision  (including  colour  perception)  must  be  normal." 

NAVY  (OFFICERS  AND  MEN). 

To  determine  the  acuity  of  vision,  Snellen's  letter  types  are 
to  be  used,  and  care  is  to  be  taken  that  the  proper  distances  are 
carefully  marked  off,  either  on  the  floor  or  on  the  wall  of  the 
room.  Should  the  room  not  be  sufficiently  long  for  the  6-metre 
card  to  be  used,  the  5-metre  card  may  be  substituted,  and  both 
these  cards  are  to  be  supplied,  as  well  as  the  proper  size  of  types 
for  testing  near  vision. 

The  colour  sense  is  to  be  determined  by  means  of  Holmgren's 
wool-test,  and  care  is  to  be  taken  that  when  the  wools  become 
dull  from  use  they  are  to  be  renewed. 

Officers. — A  candidate  must  have  no  defect  of  sight;  he  must 
be  able  to  read  without  glasses  f  by  each  eye  separately,  and  the 
near  type  at  the  distance  for  which  it  is  marked.  Squint,  or  any 
defective  action  of  the  eye  muscles,  any  disease  of  the  eye,  or 
any  imperfection  in  the  colour  sense,  disqualifies. 


VISION   TESTS  FOR  THE   SERVICES 


223 


Men  : 

Rating.  Vision  Required. 

Air  Service     . .  . .  . .     Vision,   f  one  eye,  f  the  other. 

Colour  sense  must  be  normal. 

Wiremen,   Armourer's    Crew,     Long    Service.  —  Vision,    f    both 
and  Shipwrights  eyes.     Colour   sense   must    be 

normal. 
Hostilities  only. — Vision,  ^^  both 
eyes. 
Blacksmiths,  Coopers,  Plumb- 
ers, and  Painters  . .  . .     Vision,  f  both  eyes. 

Boys    . .  . .  . .  . .     Vision,  f  both  eyes.    Colour  sense 

must  be  normal. 


Carpenter's  Crew 
Electrical  Artificers  . . 

Engine- Room  Artificers 


Vision,  f  both  eyes.    Colour  sense 
must  be  normal. 

Long  Service. — Vision,  f  both  eyes. 

Colour  sense  must  be  normal. 
Hostilities  only. — Vision,  y%  both 

eyes.     Colour  sense   must    be 

normal. 

Vision,  f  one  eye,  ^  the  other. 
Colour  sense  must  be  normal. 


Officers'  Stewards  and  Cooks,  Colour  sense  not  essential.  Glasses 
Boy  Servants,  and  Ships'  allowed.  Vision,  ^\  both  eyes. 
Cooks 


Royal  Marine  Bands  (Buglers, 
R.M.L.I.,      R.M.A.,       and 

Band  Boys) 


Royal  Marine  Artillery 


Royal  Marine  Light  Infantry 


Vision  for  Buglers:  f  one  eye, 
tV  the  other.  Colour  sense 
must  be  normal. 

Vision  for  Band  Boys:  ^  both 
eyes.  Colour  sense  not  essen- 
tial. 

Long  Service. — Vision,  ^  one  eye, 
^  the  other.  Colour  sense 
must  be  normal. 

Hostilities  only. — Vision,  ^f^  both 
eyes.  Colour  sense  not  essen- 
tial.    Glasses  allowed. 

Long  Service. — Vision,  §  one  eye, 
^  the  other.  Colour  sense 
must  be  normal. 

Hostilities  only. — Vision,  ^  both 
eyes.  Colour  sense  not  essen- 
tial.    Glasses  allowed. 


224  THE  REFRACTION  OF  THE   EYE 

Rating.  Vision  Required. 

Seamen  . ,  . ,  . .     Special    Service. — Vision,    |    one 

eye,    y\    the     other.      Colour 
sense  must  be  normal. 
Hostilities  only. — Same  standard. 

Sick- Berth  Attendants         . .     Vision,    ^    both    eyes.      Colour 

sense  must  be  normal.    Glasses 
allowed. 

Stokers  . .  . .  . .     Vision,  y\  one  eye,  ^  the  other. 

Colour  sense  not  essential. 

Any  defect  of  vision  must  be  due  to  errors  of  refraction  which 
can  be  corrected  to  normal  by  glasses,  and  vision  without  glasses 
must  in  any  case  be  not  less  than  /^  with  each  eye,  and  the  can- 
didate must  also  be  able  to  read  d  =o,  6  of  Snellen's  test  types. 

Note. — For  distant  vision,  D=f  is  considered  normal; 
for  near  vision,  ability  to  read  d  =  '6  at  any  distance  chosen 
by  the  candidate. 

Assistant  Clerkships  in  the  Navy. — Short-sighted  candidates, 
in  other  respects  fit,  are  especially  considered ;  a  moderate  degree 
of  refraction  error  would  not  disqualify,  provided  the  eyes  are 
in  other  respects  normal. 


APPOINTMENTS  UNDER  THE  GOVERNMENT  OF  INDIA. 

The  Ecclesiastical,  Education,  Geological  Survey,  Agricultural, 
Indian  Finance,  Customs,  Civil  Veterinary,  and  Other  Depart- 
ments not  specially  provided  for  in  the  following  pages. 

1.  A  candidate  may  be  admitted  into  the  Civil  Services  of 
the  Government  of  India  if  ametropic  in  one  or  both  eyes,  pro- 
vided that,  with  correcting  lenses,  the  acuteness  of  vision  be  not 
less  than  |  in  one  eye  and  f  in  the  other ;  there  being  no  morbid 
changes  in  the  fundus  of  either  eye. 

2.  Cases  of  myopia,  however,  with  a  posterior  staphyloma, 
may  be  admitted  into  the  Service,  provided  the  a,metropia  in 
either  eye  does  not  exceed  2-5  d,  and  no  active  morbid  changes  of 
choroid  or  retina  be  present. 

3 .  A  candidate  who  has  a  defect  of  vision  arising  from  nebula 
of  the  cornea  is  disqualified  if  the  sight  of  either  eye  be  less  than 
1%  ;  and  in  such  a  case  the  acuteness  of  vision  in  the  better  eye 
must  equal  |,  with  or  without  glasses. 

4.  Squint  or  any  morbid  condition,  subject  to  the  risk  of 
aggravation  or  recurrence,  in  either  eye,  may  cause  the  rejection 
of  a  candidate.  The  existence  of  imperfection  of  colour  sens*- 
will  be  noted  on  the  candidate's  papers. 


f..' 

VISION  TESTS  FOR  THE   SERVICES  22$ 

The  Departments  of  Forest,  Survey,  Telegraph,  Factories,  and  for 
Various  Artificers* 

1.  If  myopia  in  one  or  both  eyes  exists,  a  candidate  may  be 
passed,  provided  the  ametropia  does  not  exceed  2-5  d,  and  if  with 
correcting  glasses,  not  exceeding  2*5  d,  the  acuteness  of  vision 
in  one  eye  equals  f  and  in  the  other  f ,  there  being  normal  range 
of  accommodation  with  the  glasses. 

2.  Myopic  astigmatism  does  not  disqualify  a  candidate  for  ser- 
vice, provided  the  lens  or  the  combined  spherical  and  cylindrical 
lenses  required  to  correct  the  error  of  refraction  do  not  exceed 
—  2'5  d;  the  acuteness  of  vision  in  one  eye,  when  corrected,  being 
equal  to  |,  and  in  the  other  eye  f ,  together  with  normal  range 
of  accommodation  with  the  correcting  glasses,  there  being  no 
evidence  of  progressive  disease  in  the  choroid  or  retina. 

3.  A  candidate  having  total  hypermetropia  not  exceeding 
4  D  is  not  disqualified,  provided  the  sight  in  one  eye  (when 
under  the  influence  of  atropine)  equals  f ,  and  in  the  other  eye 
equals  f ,  with  +  4  d  or  any  lower  power. 

4.  Hypermetropic  astigmatism  does  not  disqualify  a  candidate 
for  the  Service,  provided  the  lens  or  combined  lenses  required  to 
cover  the  error  of  refraction  do  not  exceed  4  d,  and  that  the  sight 
of  one  eye  equals  | ,  and  of  the  other  f ,  with  or  without  such  lens 
or  lenses. 

5.  A  candidate  having  a  defect  of  vision  arising  from  nebula 
of  the  cornea  is  disqualified  if  the  sight  of  one  eye  be  less  than  y\. 
In  such  a  case  the  better  eye  must  be  emmetropic.  Defects  of 
vision  arising  from  pathological  or  other  changes  in  the  deeper 
structures  of  either  eye  which  are  not  referred  to  in  the  above 
rules  may  exclude  a  candidate  for  admission  into  the  Service. 

6.  Squint  or  any  morbid  condition,  subject  to  the  risk  of  aggra- 
vation or  recurrence,  in  either  eye,  may  cause  the  rejection  of  a 
candidate.  The  existence  of  imperfection  of  colour  sense  will 
be  noted  on  the  candidate's  papers. 

Public   Works  Department  and  Superior  Establishments, 
Railway  Department. 

1.  If  myopia  in  one  or  both  eyes  exists,  a  candidate  may  be 
passed,  provided  the  ametropia  does  not  exceed  3 '5  d,  and  if 
with  correcting  glasses  not  exceeding  3-50  the  acuteness  of  vision 
in  one  eye  equals  f ,  there  being  normal  range  of  accommodation 
with  the  glasses. 

2.  Myopic  astigmatism  does  not  disqualify  a  candidate,  pro- 
vided the  lens,  or  the  combined  spherical  and  cylindrical  lenses, 
required  to  correct  the  error  of  refraction  does  not  exceed  3*5  d; 
the  acuteness  of  vision  in  one  eye,  when  corrected,  being  equal 
to  f ,  and  in  the  other  f ,  together  with  normal  range  of  accommo- 

*  Artificers  engaged  in  map  and  plan  drawing  may  be  con- 
sidered separately,  and  this  standard  relaxed  if  it  appears  to  te 
desirable. 

15 


22  6  THE  REFRACTION   OF  THE  EYE 

dation  with  the  correcting  glasses,  there  being  no  evidence  of 
progressive  disease  in  the  choroid  or  retina. 

3.  A  candidate  having  total  hypermetropia  not  exceeding  4  d 
is  not  disqualified,  provided  the  sight  in  one  eye  (when  under 
the  influence  of  atropine)  equals  f ,  and  in  the  other  eye  equals  f , 
with  +40  glasses,  or  any  lower  power. 

4.  Hypermetropic  astigmatism  does  not  disqualify,  provided 
the  lens  or  combined  lenses  required  to  cover  the  error  of  refrac- 
tion do  not  exceed  4  d,  and  that  the  sight  of  one  eye  equals  f ,  and 
the  other  f ,  with  or  without  such  lens  or  lenses. 

5.  A  candidate  having  a  defect  of  vision  arising  from  nebula 
of  the  cornea  is  disqualified  if  the  sight  of  that  eye  be  less  than 
•j^.  In  such  a  case  the  better  eye  must  be  emmetropic.  Defects 
of  vision  arising  from  pathological  or  other  changes  in  the  deeper 
structures  of  either  eye  which  are  not  referred  to  in  these  rules 
may  exclude  a  candidate. 

6.  Squint  or  any  morbid  condition,  subject  to  the  risk  of 
aggravation  or  recurrence,  in  either  eye,  may  cause  the  rejection 
of  a  candidate.  Any  imperfection  of  the  colour  sense  is  a  dis- 
qualification for  appointment  to  the  Engineering  Branch  of  the 
Railway  Department,  or  as  Assistant  Superintendent  in  the 
Trafiic  Department.  In  all  other  cases  a  note  as  to  any  imperfec- 
tion of  colour  sense  will  be  made  on  the  candidate's  papers. 


The  Indian  Medical  Service  and  the  Police  Department. 

1.  Squint,  or  any  morbid  condition  of  the  eyes  or  of  the  lids 
of  either  eye  liable  to  the  risk  of  aggravation  or  recurrence,  will 
cause  the  rejection  of  the  candidate. 

2.  The  examination  for  determining  the  acuteness  of  vision 
includes  two  tests — one  for  distant,  the  other  for  near,  vision. 
The  army  test-types  will  be  used  for  the  test  for  distant  vision, 
without  glasses,  except  where  otherwise  stated  below,  at  a  dis- 
tance of  20  feet;  and  Snellen's  Optotypi  for  the  test  for  near 
vision,  without  glasses,  at  any  distance  selected  by  the  candidate. 
Each  eye  will  be  examined  separately,  and  the  lids  must  be  kept 
wide  open  during  the  test.  The  candidate  must  be  able  to  read 
the  tests  without  hesitation  in  ordinary  daylight. 

3.  A  candidate  possessing  acuteness  of  vision,  according  to 
one  of  the  standards  herein  laid  down,  will  not  be  rejected  on 
account  of  an  error  of  refraction,  provided  that  the  error  of 
refraction,  in  the  following  cases,  does  not  exceed  the  limits 
mentioned — viz.:  (a)  In  the  case  of  myopia,  that  the  error  of 
refraction  does  not  exceed  2*5  d;  [h)  that  any  correction  for 
astigmatism  does  not  exceed  2*5  d;  and,  in  the  case  of  myopic 
astigmatism,  that  the  total  error  of  refraction  does  not  exceed 

2-5  D. 

4.  Subject  to  the  foregoing  conditions,  the  standards  of  the 
minimum  acuteness  of  vision  with  which  a  candidate  will  be 
accepted  are  as  follows: 


VISION   TESTS   FOR  THE   SERVICES  22/ 


Standard  I. 


Right  Eye.  Left  Eye. 

Distant  vision :  V.=|.  V.=f. 

Near  vision:  Reads  o,  6.  Reads  o,  6. 


Standard  II. 
Better  Eye.  Worse  Eye. 

Distant  vision:  V.  =f.  V.,  without  glasses  =  not  below 

^;     and,     after     correction 
with  glasses  =  not  below  ^. 
Near  vision:  Reads  o,  6.  Reads  i. 


Standard  III. 

Better  Eye.  Worse  Eye. 

Distant    vision:    V.,     without  V.,  without  glasses  =  not  below 

glasses  =  not  below  -^f\  and  ^;     and,     after     correction 

after  correction  with  glasses  with  glasses  =  not  below  ^. 
=  not  below  f . 

Near  vision:  Reads  o,  8.  Reads  i. 

N.B. — In  all  other  respects  candidates  for  these  two  branches 
of  the  Service  must  come  up  to  the  standard  of  physical  require- 
ments laid  down  for  candidates  for  commissions  in  the  army. 


The  Indian  Pilot  Service,  and  Candidates  for  Appointments  as 
Guards,  Engine-drivers,  Signalmen,  and  Pointsmen  on 
Railways. 

1.  A  candidate  is  disqualified  unless  both  eyes  are  emmetropic, 
his  acuteness  of  vision  and  range  of  accommodation  being  perfect. 

2.  A  candidate  is   disqualified   by  any  imperfection  of  his 
colour  sense. 

3.  Strabismus,  or  any  defective  action  of  the  exterior  muscles 
of  the  eyeball,  disqualifies  a  candidate  for  these  branches  of 


The  Indian  Marine  Service,  including  Engineers  and  Firemen. 

1.  A  candidate  is  disqualified  if  he  has  an  error  of  refraction 
in  one  or  both  eyes  which  is  not  neutralized  by  a  concave  or  by 
a  convex  i  d  lens,  or  some  lower  power. 

2.  A  candidate  is  disqualified  by  any  imperfection  of  his  colour 
sense. 

3.  Strabismus,  or  any  defective  action  of  the  exterior  muscles 
of  the  eyeball,  disqualifies  a  candidate  for  this  branch  of  service. 


228  THE   REFRACTION   OF  THE   EYE 

Special  Duty. 

Candidates  for  special  duty  under  Government  must  possess 
such  an  amount  of  acuteness  of  vision  as  will,  without  hindrance, 
enable  them  to  perform  the  work  of  their  office  for  the  period 
their  appointment  may  last.  In  all  cases  of  imperfection  of 
colour  sense  a  note  will  be  made  on  the  candidate's  papers. 


HOME  CIVIL  SERVICE. 

There  is  no  fixed  standard.  The  candidate  is  referred  to  "  a 
competent  medical  adviser, ' '  leaving  him  to  apply  whatever  tests 
he  may  deem  suitable,  and  whatever  standard  the  particular 
situation  may  require. 

A  candidate  is  considered  unfit  if  he  has  any  serious  defect  in 
vision.  A  moderate  degree  of  ordinary  short  sight  corrected  by 
glasses  would  not,  as  a  rule,  be  regarded  as  a  disqualification; 
but  candidates  for  the  Customs  Outdoor  Service  are  liable  to 
disqualification  for  any  defect  of  vision.  Candidates  for  some 
other  appointments  of  a  special  character  would  be  rejected  for 
colour  blindness,  but  for  the  Covenanted  Civil  Service  of  India 
and  for  ordinary  home  appointments  it  is  not  by  itself  a  dis- 
qualification. 

PRISON  SERVICE. 

Candidates  are  expected  to  have  "  normal  vision  "  in  both 
eyes,  and  any  slight  departure  from  normal  vision  is  considered 
on  its  merits  in  accordance  with  the  duties  which  the  candidate 
would  be  required  to  perform  if  appointed. 


THE  METROPOLITAN  POLICE  SERVICE. 

Candidates  are  required  to  have  "normal  vision"  in  both 
eyes,  without  glasses.  The  ordinary  test-types  are  used,  and 
the  range  of  accommodation  is  sometimes,  but  not  uniformly, 
tested.  Candidates  who  show  only  slight  deviation  from  the 
**  normal  "  standard  are  considered  on  their  merits. 


ENGLISH  RAILWAYS. 

No  uniform  standard.     Each  company  has  its  own  standard. 
Every  engine-driver  should  have  normal  colour  perception; 
and,  without  glasses,  vision  should  be  at  least  ^  in  each  eye. 


VISION   TESTS   FOR  THE   SERVICES 


229 


en 
1 

Vision     with     glasses 
(corrected      vision) 
counts. 

2i 

> 

en 

.d 

li 

en  y 

en 

<u 

"to 
.d 

"^e/5 

li 

■J)  0 
•> 

Vision  without  glasses 
counts.     For    home 
service,  garrison  ser- 
vice,   and    garrison 
service  abroad,  glas- 
ses are  allowed  with- 
in unspecified  limits. 

J 

Ul 

> 

1 

•E 

a 

in 

1 
1 

1 

i 

0  <=> 

o.d 

.S  d 
H* 

a;  0 
d  0 

+-> 

.Sd 

Ul  S 
0    3 

^* 

.So 

H« 

U  ncor r e  cted 
vision  must  be 
I  in  better  eye, 
^(j-  in  worse  eye. 
The  better  eye 
may  be  the  left. 

1 

\  in  better  eye.    Other 
eye  may  have  mini- 
mal    vision.        For 
Landsturm      vision 
=  \.     If  one  eye  has 
vision  =  ^  the  other 
may  be  blind. 

Group    I,    J   in    each 
eye.     Group  2,  ^  in 
one ;  ^  in  other. 

d 

II 

H« 

No  correction  allowed 
for   general   service. 
Uncorrected     vision 
must   be  ^  in  each 
eye,  or  4  in  the  right 
eye  with  ,V  in  the 
left. 

< 

1 

1 

Ui 

z 

c 

.a 

8 

1 

Above  6  D.  no  limit  if 
standard  of  corrected 
vision  is  attained. 

Above  7  D.  no  limit  if 
standard  of  corrected 
vision  is  attained. 

1 

No  amount  specified, 
but      according      to 
vision  required  high- 
est amount  possible 
is   about    2-5    D.    in 
better  eye  and  3'5D. 
in  worse  eye. 

1 

6-5  D.    For  Land- 
sturm no  limit 
if   standard    of 
corrected  vision 
attained. 

0 

Q 

No  amount  speci- 
fied, but  accord- 
ing    to     vision 
required  highest 
amount  possible 
is  about  2*5  D. 

Germany 

< 

H 

D 
< 

0 
< 

H 
1— 1 

Great 
Britain* 

BIBLIOGRAPHY 

I  HAVE  to  acknowledge  my  indebtedness  to  the  follow- 
ing books  and  papers  for  much  valuable  material : 

"  Accommodation  and  Refraction  of  the  Eye  ":  Donders. 
"  The  Refraction  and  Accommodation  of  the  Eye  ":  Landclt. 
"  Refraction  of  the  Eye  " :  Hartridge. 
"  Refraction  and  how  to  Refract  ":  Thorington. 
"  The  Eye,  its  Refraction  and  Diseases  ":  Gibbons. 
"Refraction":  Druiff. 
"  Kurzsichtigkeit  " :  Hirshberg. 

"  Anomalies  de  la  Refraction  ":  Fromaget  and  Bichelonne. 
"  Squint  " :  Worth. 

"  Convergent  Strabismus  ":  Holthouse. 
"The  Ocular  Muscles":  Maddox.     2nd  edition. 
"  Ophthalmological  Prisms  ":  Maddox. 
"  Prismatic  Combinations  ":  Percival. 
"  Prescribing  of  Spectacles  ":  Percival. 
"  Examination  of  the  Eye  ":  Snell. 
"  Physiologic  Optics  ":  Tscherning. 
"  Elementary  Ophthalmic  Optics  ":  Parsons. 
"  Textbook  of  Physiology'  ":  Michael  Foster. 
"  Light  " :  Lewis  Wright. 
"  Treatise  on  Practical  Light  ":  R.  S.  Clay. 
"  Curiosities  of  Light  and  Sight  ":  Shelford  Bidwell. 
"  Hygiene  of  the  Eye  " :  Cohn. 
"  Glaucoma  ":  Priestley  Smith. 

"  Textbook  of  Ophthalmology  ":  Fuchs.     3rd  edition. 
"  Diseases  of  the  Eye  " :  Fick. 
"  Diseases  of  the  Eye  ":  Berry. 
"  Diseases  of  the  Eye  ":  Mayou.     2nd  edition. 
"  Diseases  of  the  Eye  " :  Swanzy. 
230 


BIBLIOGRAPHY  23I 

"  System  of  Diseases  of  the  Eye  ":  Norris  and  Oliver. 

"  Diseases  of  the  Eye  " :  De  Schweinitz.      8th  edition. 

"  Diseases  of  the  Eye  ":  Noyes. 

"  Handbuch  der  Augenheilkunde  " :  Graefe  and  Saemisch. 

"  Precis  d'Ophthalmologie  " :  Morax. 

"  Functional  Nervous  Diseases  ":  Stevens. 

"  Diseases  of  the  Nervous  System  ":  Gowers. 

"  The  Diseases  of  the  Nervous  System  " :  Ross. 

"  Lectures  on  Nervous  Diseases  ":  Ranney. 

"  The  Eye  in  General  Disease  " :  Knies. 

"  Minor  iVIaladies  " :  Leonard  Williams. 

"Headache  and  other  Morbid  Cephalic  Sensations":  Harry 
Campbell. 

"Megrim  and  Sick  Headaches":  Liveing. 
"Headaches":  Day. 

Transactions  of  the  Ophthalmological  Society. 

The  Ophthalmoscope. 

The  Ophthalmic  Review. 

British  Journal  of  Ophthalmology. 

Graefe' s  Archiv  f.  Ophthalmologie. 

Knapp's  Archives  of  Ophthalmology. 

Transactions  of  the  A  merican  Ophthalmological  Society. 

A  merican  Journal  of  Ophthalmology. 

Annals  d'Oculistique. 


INDEX 


ABDUCTING    power     of    external 

rectus,  178 
Aberration,  chromatic,  133 

in   bi-focal  lenses,    18 
spherical,  18 

of  crystalline  lens,  34 
Absolute  hyperopia,  90,  93 
Accommodation,  29 
absolute,  38 

amplitude  of,  34,  88,  loi,  142 
and  convergence,  38,  50,  52, 

lOI 

asymmetrical,  123,  154 

binocular,  38 

centre  of,  42 

cramp  of,  94,  105,  201,  218 

diminution  of,  142 

far  point  of,  34,  85,  100 

fatigue  of,  151 

influence  of  age  upon,  142 

in  astigmatism,  123 

in  hyperopia,  87 

in  myopia,  100 

in  presbyopia,  142 

iris  in,  23 

loss  of,  142,  148 

mechanism  of,  31 

muscle  of,  33 

near  point  of,  31,  34,  88,  loi 

paralysis  of,  by  cycloplegics, 

region  of,  37 

relative,  38 

spasm  of,  94,  105,  201,  218 

theories  of,  33 

theory  of  squint,  187 
Acquired  hyperopia,  92,  145 
Acuity  of  vision,  25 

in  absolute  hyperopia,  92 


Acuity  in  astigmatism,  ii6,  118 

in  myopia,  105 

record  of,  26,  207 
Adducting     power     of     internal 

rectus,  175 
Age,   influence  of,   upon  accom- 
modation, 142 
Ague,  brow,  164 
Air,  Royal,  Service,  vision  tests 

for,  222 
Alpha,  angle,  187 
Alternating  strabismus,  185 
Amblyopia,  157,  190,  191 

as  a  cause  of  squint,  191 

ex  anopsia,  157,  191 
Amblyoscope,  Worth's,   195 
Ametropia,  24 

axial,  115 

curvature,  116 
Amplitude     of     accommodation, 
34,  88,  loi,  142 

of  convergence,  50,  182 
Anatomical  conditions  associated 
with  myopia,  103,  104 

with  strabismus,  190 
Angle  alpha,  187 

gamma,  95,  103,  105,  187 

of  convergence,  48 

of  deviation,  5 

of  five  minutes,  26 

of  incidence,  57 

of  one  minute,  25 

of  prism,  5 

of  reflection,  57 

of  refraction,  5,  7 

of  strabismus,  192 

of  vision,  25 
Anisometropia,  152,  214,  216 

asymmetry  of  face  in,  153 


232 


INDEX 


233 


Anisometropia,  binocular  vision 
in,    153 

varieties  of,  153 
Annular  muscle  of  Miiller,  23,  95 
Anterior  linear  focus,  117 

principal  focus,  20 
Antimetropia,  152 
Aphakia,  16,  92,  159,  219 
Apparatus  for  vision  testing,  202 
Apparent   myopia,    89,   94,    109, 
218 

strabismus,  95,  105,  187,  189 
Army,  vision  tests  for,  221 
Artificial  light  in  eye-work,  63, 

66,  202,  203 
Asthenopia.     See  Eyestrain 
Astigmatic  clock,  132 

fan,  122,  132 

headache,  122,  163 

hyperopia,  119,  214 

myopia,  119,  215 

surface,   images  formed  by, 
117 
Astigmatism,   115 

accommodation  in,  123 

asymmetric,  120 

asymmetrical        accommoda- 
tion in,   123 

compound  hyperopic,  119 
myopic,  119,  215 

corneal,  121,  125,  126,  139 

crystalline,  121,  125 

diagnosis  of,  125 

direct,  119 

form  of  images  in,  117 

homonymous,  120 

hyperopic,  119,  203 

inverse,  119,  160 

irregular,  139 

lenticular,  121 

measurement  of,  72,  125 

mixed,  119,  137,  216 

myopic,  119,  215,  216 

oblique,  73,  80,  119,  131,  216 

physiological,  irregular,  139 
regular,  121 

regular,   118 

seat  of,    121 

simple   hyperopic,    119 

small  errors  of,  relative  fre- 
quency of,  135 

symmetric,  119 


Astigmatism,  tests   for,    125 

transient,  121 

treatment  of,  135 
Asymmetric  astigmatism,  120 
Asymmetrical  accommodation  in 
astigmatism,  123 
in  anisometropia,   154 
Asymmetry  of  face  in  anisome- 
tropia, 153 
in    astigmatism,    125 
Atrophy,  choroidal,   in  myopia, 

107 
Atropine,  97,  iii,  135,  198 

tabloids  or  discs,  200 
Attachments  of  eye-muscles,  40 
Axial  ametropia,  115 

hyperopia,  91 

myopia,  100 
Axis,  major,  of  cornea,  187 

of  cylinders,  13,  136,  138 

of  rotation  of  eye-muscles,  40 

optic,  19,  22,  187 

principal,  9 

secondary,  9 

visual,  187 

Band  of  light  in  retinoscopy,  80 
Base  of  prism,  3,  5 
Bi-concave  lens,  9,  11 
Bi-convex  lens,  8,  10 
Bi-focal  lenses,  16,  151,  201 
Bilious  headache,  165 
Binocular  accommodation,  38 

in  anisometropia,  154 
vision,  42 

loss  of,  in  hyperopia,  91 

tests  for,  153 
Blepharitis,  92,  163 
Blinking  and  eyestrain,  167 
Brain-fag,  167 
Bridge  of  spectacles,  209 
Brow-ague,  164 

Camera,  photographic,  19 

Capsule  of  lens,  ^3 
Tenon's,  39 

Cardinal  points,  20 

Cases,  illustrative,  213 

Cataract,  125,  163 

caused  by  eyestrain,  125,  163 
extraction,  glasses  after,  159 


234 


THE   REFRACTION   OF   THE    EYE 


Cataract,  increase  of  density  of 
lens,  causing  myopia,   loi 
monocular  polyopia  in,    139 
Centrad,  6 

Central  motor  asthenopia,  173 
Centre,  oculo-motor,  42,  43 
of  accommodation,  42,  43 
of  internal  rectus,  42,  43 
of  rotation,  188 
optic,  43 
optical,     of     lenses,     9,     22, 

208 
pupillary,  42,  43 
Centring  of  lenses,  208 

method  of  checking.  209 
Changes   in  fundus   in  myopia, 

107,  113 
Chart     of      accommodation, 
author's,  147 
Bonders',  143,  144,  145 
Choreiform  movements  of  facial 

muscles,  166,  167 
Choroidal    atrophy    in    myopia, 

107 
Chromatic  aberration,  133 
Chromo-aberration  test,  133 
Ciliary  muscle,  32,  ^;^ 
fatigue  of,  151 
in  hyperopia,  33,  95 
in  myopia,  ^3)  112 
processes,  3^ 
spasm  of,   94,   105,  201, 
218 
Circles,  diffusion,  30,   105,   107 
Circular    muscle   of    Miiller,    33, 

95 
Civil    Service,    vision    tests    for 
Home,  228 
Indian,  224 
Clock-face,  132 

Cobalt-blue  glass  test  for   astig- 
matism, 133 
Cocaine,  199 

Combination  of  lenses,  15 
Compound    hyperopic    astigma- 
tism, 119,  214 
myopic  astigmatism,  119,215 
Concave  lenses,  9,  11,  16,  99,  176, 
203,  208 
mirror,  57,  59,  82 
Concomitant  squint,  184 
Confusion  letters,  133 


Confusion  of  images  in  astigma- 
tism, 118 
Congenital  defect  of  eye-muscles, 
i8i 
tendency  to  myopia,  104 
Conical  cornea,  loi,  141 
Conjugate  foci  in  lenses,  ij 

in  mirror,  58 
Conjunctivitis     and     eyestrain, 

92,  163 
Constant  squint,  185 
Convergence,  39,  48 

amplitude  or   range  of„  50, 

172 
and  accommodation,  38,  52, 

90,   lOI 
angle  of,  48 
insufficiency  of,  53,  105,  172, 

175,  220 
latent,  44,  177 
positive   part    of    amplitude 

of,  172 
punctum  proximum  in,  51 

remotum,  50 
relation  between,  and  accom- 
modation, 38,  52,  90,  lOI 
Convergent   strabismus,    91,    95, 

184,  187,  193,  220 
Convex  lenses,  8,  10,  203,  208 
Cornea,  21,  31,  121 
conical,  loi,  141 
in  hyperopia,  95 
ulcers  of,  163 

causing  astigmatism,  139 
Corneal     astigmatism,     regular, 
115,  121,  123 
irregular,  139 
axis,  187 
Cramp    of    accommodation,    94, 

105,  201,  218 
Crescent  in  myopia,  107 
Crookes's  glass,  211 
Crossed  diplopia,  184 
Crown  glass,  index  of  refraction 

of,  4 
Cupping  of  myopic  disc,  107 
Curvature,  ametropia  of ,  116 

hyperopia,  92 
Customs  Outdoor  Service,  vision 

required  for,  228 
Cyclophoria,   180 
Cycloplegia,  198 


INDEX 


235 


Cycloplegics,  198 
Cylindrical  lenses,    12, 
204 

axis  of,  12,  136 

testing,  14 


16,    135, 


Dark  room,  202 

Decentring  lenses,  178 

Desks  in  schools,  height  of,  no 

Detachment   of    retina    in   high 

myopia,  108,  113 
Deviation,  angle  of,  in  prisms,  5 
latent,   of   muscles,   44,    105, 

172 
primary,  in  strabismus,  192 
secondary,     in     strabismus, 
192 
Diaphragm  test,  Harman's,  183 
Diffusion  circles,  30,  107 
Dioptre,  13 
prism,  6 

table  of,  and  inches,  14 
Dioptric  apparatus  of  the  eye,  19 

system,   13 
Diplopia,  183 

heteronymous,  184,  186 
homonymous,  184,  185 
Direct     ophthalmoscopic     exam- 
ination, 66 
Disc,  optic,  66,  95,  106 

cupped    or    "  dragged,"    in 

myopia,  107 
shape  of,  in  astigmatism,  65 
in  myopia,  106,  107 
Disc,  pin-hole,  18,  141,  204 
Placido's,  140 
stenopaic,  141,  204 
Discission     of     lens     in     high 

myopia,  114 
Dissociation    between    accommo- 
dation   and    convergence,    38, 
54,  90,  102 
Distance,  test  type  for,  26,  202 
Distant  vision,  Maddox  test  in, 

relation  of  eyes  in,  44 
Divergence,  latent,  105,  171,  175 
Divergent  strabismus,   105,   184, 

197 
Dixey  glasses  for  anisometropia, 

155 
test  types,  202 


Donders'   formula  for  accommo- 
dation, 34 
for  convergence,  50 
on  astigmatism,  123 
on   convergence   in   myopia, 

103,  108 
on  presbyopia,  146,  148 
Dot  and  line  test  (Graefe's),  54 
Dynamic  lenticular  astigmatism, 
121 
equilibrium   of   ocular   mus- 
cles, 48 
Dynamometer,  Landolt's,  52 
Dyspepsia  due  to  eyestrain,  169, 

Elasticity  of  lens,  diminution  of, 

with  age,  142,  147 
Electric  ophthalmoscope,  61 
Elongation  of  eyeball,  25,  100 
Emmetropia,  24,  35,  64,  68,  79, 

142,  180 
Epilepsy,   relation   between  eye- 
strain and,  166,  214 
Erect  image  in  high  hyperopia, 

65 
in  spherical  lenses,  n 

Eserine,  200,  205 
Esophorio,  176,  219 
Esotropia,  184 
Examination,  methods  of,  202 

of  cornea,  63 

of  patients,  205 

ophthalmoscopic,  63 
Exercises,  orthoptic,  181 
Exophoria,  175 
Exotropia,  184 
External  rectus  muscle,  39 

abducting  power  of,  178 

insufficiency  of,  176 

tenotomy  of,  182,  197 
Eye,  normal,  optic  properties  of, 

19 
standard,  21 
Eyestrain,  astigmatism  and,  122, 

136 
blepharitis  from,  92,  163 
brain-fag  and,  167 
choreiform  movements  from, 

^67       .   .  .         ^ 
conjunctivitis  and,  92 
definition  of,  162 


236 


THE   REFRACTION   OF   THE   EYE 


Eyestrain,  dyspepsia  from,   169, 

epilepsy  and,  166 
headache  and,  122,  163 
heterophoria  and,  173 
hyperopia  and,  "6"],  93 
insomnia  and,  168 
migraine  and,  164 
nerve  exhaustion  and,  167 
neurasthenia  and,  167 
presbyopia  and,  149 
shell  shock  and,  170 

Face,    asymmetry    of,    in    aniso- 
metropia, 153 
Facultative  hyperopia,  90,  92 
Fan,  122 

Far  point  of  accommodation,  34 
in  hyperopia,  85 
in  myopia,  100 
of  convergence,  50 
Fatigue  of  accommodation,  151 
P^ocal  distance  of  lenses,  10 
illumination,  63 
interval  of  Sturm,  118 
Focus,  anterior,  20 

conjugate,  in  convex  lenses, 
II 
in  concave  mirrors,  57 
linear,  in  astigmatism,  117 
posterior,  20 
principal,   10,   11 
virtual,  11 
Focus-glass,  62 
Folders,  210 

Formation   of   images   by   astig- 
matic surfaces,  117,  118 
in  indirect  ophthalmoscopy, 

64 
in  lenses,  10 
in  the  eye,  22 
Frames,  spectacle,  210 

scholar's,  210 
Franklin  lenses,  17 
Frequency,  relative,  of  different 
forms  of  ametropia,  25 
of  small  errors  of  astig- 
matism, 135 
Fundus   in  myopia,    107 
in  astigmatism,    131 
in    direct    method    of    oph- 
thalmoscopy,  67 


Fundus    in    indirect   method   of 

ophthalmoscopy,    64 
Fusion   sense,    191,    196 

supplement,   54,   102,   171 

Gamma,   angle,   95,    105,    187 

Glass,  focus,  62 
Crookes's,  211 

Glasses.     See  Lenses 

coloured,    Snellen's,   43,    153 
dark  or  tinted,   113,  204 

Glass-rod  test,  44 

Glaucoma   caused  by   eyestrain, 

163 
in  hyperopia,  98 
caused  by  atropine,  98,  200 
Gould's   biographic  clinics,    170 
Graefe's     explanation     of     con- 
vergence  of   covered   eye, 
42 
dot  and  line  test,  54 

Hamblin,    scholar's    frame,    210 

trial  case,  204 
Harman's    diaphragm    test,    183 
Harwood,  Dr.,  on  eyestrain,  170 
Headache,  bilious,    165 

in  astigmatism,   122 

ocular,    163 
Helmholtz's     theory    of     accom- 
modation, 33 

ophthalmoscope,   58 
Hemicrania,  164 
Hereditary  tendency  to  myopia, 

104 
Hess,  :i2> 

Heteronymous   diplopia,    184 
Heterophoria,   171,  213 

cause  of,  171 

eyestrain  and,   173 

tenotomy  for,   182 

tests  for,  44,   175,  177 

varieties  of,  174 
Heterotropia,   174,   184 
High  myopia,  107,  112,  113,  212 

discission  of  lens  in,  114 

treatment  of,    112 
Homatropine,   97,    112,   199 
Home  Civil  Service,  vision  tests 

for,  228 
Homonymous    astigmatism,    120 

diplopia,   184 


INDEX 


237 


"  Hook-fronts,"  151 
Hygiene,  ophthalinic,   no 
Hyperesophoria,   180 
Hyper  exophoria,   180 
Hyperopia,  25,  85,  213 

absolute,  90,  93 

accommodation  in,  35,  88 

acquired,  92,  145 

angle  gamma  in,  95,  187 

axial,  91 

causes  of,  91 

curvature,  92 

diagnosis  of,  95 

esophoria  and,   178 

estimation  of,   95 

exophoria  and  high,   175 

eyestrain  in,  92 

facultative,  90,  92 

index,  92 

latent,  90 

length  of  eye  in,  91 

manifest,  89 

physical  signs  in,  95 

punctum   proximum   in,   35, 
88 
remotum  in,  35,  85 

relative,  91 

symptoms  of,  92 

total,   90 

treatment  of,  95 

varieties  of,  91 
Hyperopic  astigmatism,  119,  214 
Hyperphoria,   158,   179 
Hypertropia,  185 

Illumination,  focal,  63 

in  retinoscopy,  78 

of  dark  room,  202 

of  test  types,  203 
Images  crossed  in  diplopia,  184 

erect,   65,   95 

form    of,     in    astigmatism, 
117,  118 

formation  of,  in  the  eye,  22 
in  indirect  method,  64 
in  lenses,  11 

in   emmetropia,    64 

in  hyperopia,  65,  95 

in  myopia,  65 

inverted,  64 

on  cornea,  31 

on  crystalline  lens,  31 


Images,  retinal,  size  of,  23 

virtual,   11 
Inch      system       of      measuring 
lenses,  13 
compared  with  dioptres,    14 
Incidence,  angle  of,  57 
Incipient     cataract,     monocular 

polyopia  in,   139 
Index  hyperopia,  92 
of  refraction,  3 
Indian     Civil     Service,     vision 
tests  in,  224 
Marine  Service,  vision  tests 

in,  227 
Medical       Service,       vision 

tests  in,  226 
Pilot    Service,    vision    tests 

in,  227 
Police   Service,    vision  tests 

in,  227 
Railways,     vision    tests    in, 
225,  227 
Indirect   method    of    ophthalmo- 
scopic  examination,   63 
advantages  of,  65 
Infinity,    punctum   remotum    at, 
in  accommodation,  35 
in  convergence,  50 
Influence  of  age  upon  accommo- 
dation, 142 
Initial  convergence,  54 
Insomnia,    168,   214 
Insufficiency      of      convergence, 
53>  i7i>  175.  181,  220 
of     eye-muscles,     102,     171, 
172,  175,  176 
Intermittent  squint,  186 
Internal  rectus,  39,  41 

adducting,  power  of,  175 

centre  of,  43 

insufficiency     of,     53,     102, 

175 

tenotomy  of,  in  strabismus, 
196 
Interval,  focal,  of  Sturm,   118 
Inverted  image,  64,  65 
Iris,  32,  33 

constrictor  of,  42 

in   accommodation,   33 
Irregular  astigmatism,  139 

of  cornea,  139 

of  lens,  139 


238 


THE    REFRACTION    OF   THE   EYE 


Jaeger  test  type,  28 

J  aval's  ophthalmometer,   126 

Lagging  of  convergence  behind 

accommodation,    52 
Landolt  on  convergence,   42,   56 
on  "motor  asthenopia,"  173 
on     tenotomy      for      hetero- 
phoria,  182 
Landolt' s       ophthalmq- dynamo- 
meter, 51,  52 
Latent  convergence,  47,  176 
deviation,  44,  171 
divergence,  47,  52,  175 
hyperopia,  90 
Length  of  eyeball,  21 
Lens,  crystalline,  30 

absence  of,  in  aphakia,   159 
capsule  of,  3;^ 

changes    in,    during    accom- 
modation, 30,  31,  32 
discission  of,  in  high  myo- 
pia,  114 
elasticity  of,   33 

diminished       by       age, 
142,   147 
images  on,  31 
irregular     astigmatism     of, 

139 

regular  astigmatism  of,  121, 
123 

spherical   aberration   in,    18 
Lens,  magnifying   for  examina- 
tion, 62 

Voigtlaender's,  63 
Lenses,  bi-concave,  8,  ii,  15,  99, 
176,  203 

bi-convex,  8,   10,  15,  86,  96, 
203 

bi-focal,  16,  151,  161 

centring  of,  208 

combination  of,  15 

concavo-convex,  9 

Crookes's,  211 

cylindrical,   12,  16,   135,  203 

decentred,  176,  178,  208 

Franklin,  17 

meniscus,  9 

neutralizing,   14 

numeration  of,  13 

periscopic,  15,  137 

plano-concave,  9 


Lenses,  plano-convex,  9 

spherical,  9 

sphero-cylindrical,    15 

testing,   14,  209 

tinted,  113,  204 

toric,  16,   160 
Lenticular      astigmatism,       121, 

123,  139 
Letters  of  test  types,  26 

Pray's,  134 
Light,  velocity  of,  i 

artificial,  63,  203 
Linear     focus     in    astigmatism, 

117 
Liveing    and    "  nerve    storms," 

165 
Luxe  bi-focals,  17 

Macula,  72 

changes  in,  in  myopia,  107 
Maddox  test  for  muscles,  44,  54 
Malignant  myopia,  108 
Malingering,   183 
Manifest  hyperopia,   89 
Marple  mirror,  62 
Marple's  skiascopes,  83 
Measurement        by        "direct 
method,"  66 

of  astigmatism,  72,   130 

of  hyperopia,  70,  95 

of  myopia,  69,  106 
Mechanism    of    accommodation, 

31 

Media,  refracting,  of  the  eye, 
20 

Meniscus,  9 

Meridians,  principal,  in  astig- 
matism, 116,  119,  136 

Meridional  asymmetrical  ac- 
commodation, 123 

Metre  angle,  49 

Metrical  system  of  measuring 
lenses,  13 

Migraine,   165 

Mires  of  ophthalmometer,  127, 
129 

Mirror,    Marple's,   62 

Mirrors,  centre  of  curvature  of, 

concave,   57,  82,  205 
plane,  57,  82,  205 
reflection  from,  57 


INDEX 


239 


Mixed    astigmatism,     119,     137, 

216 
Monocles,  157,  211 
Monocular  polyopia  in  cataract, 

139 

vision,  43,  157 
Monolateral  strabismus,  185 
Morton's  ophthalmoscope,  60 
Motor  asthenopia,  central,    173 

peripheral,  173 

centres,  42,  43 
Miiller,  annular  muscle  of,  33,  95 
Muscae  volitantes,  105 
Muscle,  ciliary,  ^^ 

spasm,  94,  105,  201,  206 

strain,  172 
Muscles  of  eyeball,  39,  40 

attachments  of,  40 

axis  of  rotation  of,  40 

congenital  defect  of,  181 

equilibrium  of,  48 
Muscular  inefficiency,  173 

insufficiency,  172 
Mydriasis,      causing      spherical 

aberration,  18 
Mydriatics,  199 
Myopia,  25,  99,  213,  217 

accommodation  in,  loi 

apparent,  89,  94,  109,  218 

axial,   100 

causes  of,  104 

ciliary  muscle  in,  ^;i,  112 

detachment    of     retina    in, 
108,  113 

diagnosis  of,  105 

exophoria  and,  175 

formation   of   image   in,   by 
indirect     ophthalmoscopy, 

65 

full  correction  of,   io8,   109, 
219 

hereditary  tendency  to,   104 

high,  107,  112,  218 

discission  of  lens  in,  114 
treatment  of,  112 

influence  of  age  on,  loi 

length  of  eyeball  in,  loi,  104 

malignant,  108 

measurement  of,  by  ophthal- 
moscope, 69,   106 

ophthalmoscopic  appearances 
in,  107 


Myopia,  optic  disc  in,  107 

posterior      staphyloma      in, 

100,  107 
presbyopia  and,   144 
progressive,  108,  iii 
punctum   proximum   in,   35, 

lOI 

remotum  in,  35,  99 
refractive,   100 
region     of     accommodation 

in,  38 
retinoscopy  in,  74 
shape  of  eyeball  in,  103 
symptoms  of,   105 
treatment  of,  108 
visual  acuteness  in,  105 
vitreous  opacities  in,  105 
Myopic    astigmatism,    119,    215, 

218 
crescent,   107 

Nagel's  metre  angle,  49 
Navy,  vision  tests  for,  222 
Near    point    of    accommodation, 
3i>  34,  88,   loi 

convergence,   51 
Negative  aberration,  18 

angle  alpha,   187 

gamma,  105,  187 
Nerve  centres,  ocular,  42 

optic,  43 

power  waste,  167 
Neurasthenia,   167 
Neurasthenic    muscular     insuffi- 
ciency, 175,  183 
Neutralizing  lenses,  14 
Nodal  point,  20,  21,  25 
Normal  vision,  27 

relation  of  eyes  in,  44 
Note-book,  205 
Note-taking,  205 
Numbering  of  lenses,  13 

of  prisms,  5 

Oblique    astigmatism,    80,    119,. 

126 
Obscurations  of  vision,  94,   149 
Ocular  headache,   163 

muscles.  See  Muscles 
Oculomotor  centre,  42,  43 
Operative    treatment    of    hetero- 

phoria,  182 


240 


THE   REFRACTION    OF   THE    EYE 


Operative  treatment  of  myopia, 
114 
of  strabismus,   196 
Ophthalmic    discs    or    tabloids, 
200 
hygiene,  no 
Ophthalmo-dynamometer,  52 
Ophthalmometer,  Hardy's,  130 

Meyrowitz's,  126 
Ophthalmoscope,  58 
electric,  61 
Morton's,  60 
qualities  of  a  good,  59 
Ophthalmoscopic      examination, 

63 

in  astigmatism,  130 

in  hyperopia,  95 

in  myopia,   106 
Optic  axis  of  lenses,  14,  19 

centre,  43 

disc,  65,  95,  106,  107 
Optical  centre  of  lenses,   9,    19, 

208 
Optics,  I 

of  reflection,  57 
Orbit,  shape  of,  in  myopia,  103 
Orthophoria,  47,  174 
Orthoptic  exercises,  181 

Paralysis  of  accommodation  by 
cycloplegic,  95,  109,  193,  198 

Perimeter,   192 

Periodic  squint,  186 

Periscopic  lenses,  15,  137,  210 

Photographic  camera  compared 
to  eye,  19 

Physiological   astigmatism,    121, 

139 
Pilot  Service,  Indian,  tests  for, 

227 
Pince-nez,  211 
Pin-hole  disc,  18,  141,  204 
Placido's  disc,  140 
Plano-concave  lens,  9 

convex  lens,  8 
Point,    far.      See    Punctum    re- 
motum 
near.      See  Punctum  proxi- 

mum 
of    reversal    in    retinoscopy, 

78 
Points,  cardinal,  20 


Points,  nodal,  20 
principal,  20 
Police,  Indian,  test  for,  226 

Metropolitan,  228 
Polyopia,    monocular,    in    catar* 

act,  139 
Positive       angle       alpha       and 
gamma,  187 
convergence,  172 
Pray's  letters,  133,  134 
Presbyopia,  142 

accommodation  in,  142 
anisometropia  and,   157 
definition  of,    142,    148 
eyestrain  in,    149 
symptoms  of,   148 
tables     for     accommodation 

in,   146,   147 
treatment  of,  149 
Presbyopic  point,  148 
Prescription   forms   for    glasses, 

138,  205  ^         _ 

Primary    deviation    in    strabis- 
mus,  192 
Principal  axis,  9 
focus,  10,  20 

meridians    in    astigmatism, 
116 
axis  of,  119 
plane,  22 
points,  20 
Prism  dioptre,  6 
rotary,  7 

test  for  binocular  vision,  153 
test  for  malingering,  183 
Prisms,  5 

action  of  light  on,  2,  5 
angle  of,  5 

deviation  of,    5 
measurement     of,     by     cen- 

trads,  6 
numbering  of,   :; 
rotating,  7 

uses  of,  7,  176,  178,  182,  183 
Prison  service,   vision   required 

for,  228 
Progressive  myopia,  108,  in 
Projection,  visual,  77 
Prominence  of   eyeball   in  myo- 
pia,  105 
Public    v^orks,    Indian    Service, 
vision  tests  for,  225 


INDEX 


241 


Punctum    proximum    of    accom- 
modation, 31,  34 
in  emmetropia,  35 
in  hyperopia,  35,  88 
in  myopia,  35,  loi 
of  convergence,  51 
Punctum  remotum  of  accommo- 
dation, 34 
in  hyperopia,  35,  85 
in  myopia,  35,  99 
of  convergence,  50 
Pupil,     contraction    of,     hiding 
aberration,  18,  34 
in  accommodation,  34,  42 
in  hyperopia,  94 
in  myopia,   105 
Pupillary  centre,  42 

Qualities   of    a    good   refraction 
ophthalmoscope,  59 

Railways,    vision    tests    for    ser- 
vice on  English,  228 

Indian,  225,  227 
Range    of    accommodation    and 
convergence.      See   Amplitude 
Rectus  externus,  39,  40,  41,  176, 
182 

abducting  power  of,  178 

insufficiency  of,   176,   182 

tenotomy  of,  in  strabismus, 
197 
Rectus  internus,  39,  41,  175 

adducting  power  of,   175 

insuflSciency  of,  175,  181 

tenotomy  of,  in  strabismus, 
196 
Reduced  eye,  21 
Reflection,  angle  of,  57 

by  mirrors,  57 
Refraction  of  light,  i 

angle  of,  in  prisms,  5,  7 

by  lenses,  8 

by  prisms,  3,  5 

by  spherical  surfaces,  19 

by  the  eye,  20 

explanation  of,  2 

index  of,  3 
Region  of  accommodation,  37 
Regular  astigmatism,   116 
of  lenSj  i2ij   123 


Regulations    for    use    in    testing 

for  Services,  222 
Relation  between  accommodation 

and  convergence,   38,    52,    90, 

lOI 

Relative  accommodation,  38 

frequency  of  different  forms 
of  ametropia,  25 
of  small  errors  of  astig- 
matism,   135 
hyperopia,   91 

range      of      accommodation 
and  convergence,  52 
Remotum,   punctum.     See  Punc- 
tum 
Retina,  19,  72 

exudations    or    tumours    of, 

causing   hyperopia,   92 
in  myopia,  107 
Retinal   detachment   in   myopia, 
108 
image,  size  of,  23 
Retinoscopy,  74 

Reversal,    point    of,    in    retinos- 
copy,  78 
Reversible  spectacles,  158 
Risley's  rotary  prism,  7 
Rod  test,  Maddox,  44,  45 
Room,  dark,  203 
Rotating  prisms,  7 
Rotation,    axis    of,    of   eye    mus- 
cles, 39,  40 
Royal  Air   Service,   vision   tests 
in,  222 

Scale,      Maddox,      for      distant 
vision,  44,  45 
for  near  vision,  45,  53 
Scholar's  frame,  210 
Schools,       ophthalmic       hygiene 

in,   no 
Scissors    movement    in    retinos- 
copy, 80 
Sclerotic,  23 

in  myopia,  104 
Secondary  axes,  9 

deviation      in      strabismus, 
192 
Shadow  test,  74 
Shape  of  eyeball   in   hyperopia, 

95 
in  myopia,  103 

16 


242 


THE    REFRACTION    OF    THE    EYE 


Shell  shock,  170 
Short-sight.     See  Myopia 
Simple    hyperopia    astigmatism, 
119 
myopic     astigmatism,      119, 

Skiascope,  Marple's,  83 
Skiascopy,  74 

Snellen's  coloured  glass  test,  43, 
153.  183 

type,  26,  202 
Spasm    of    accommodation,    94, 

loi,  105,  201,  218 
Spectacles,  208 

bi-focal,  16 

bridge  of,  209 

for  aphakia,   161 

for      regular      astigmatism, 

for    irregular    astigmatism, 
141 

for  myopia,   iii 

for  presbyopia,  150 

frames,  209 

Franklin,  17 

reversible,  158 

scholar's  frame,  210 

stenopaic,   141 
Spherical  aberration,    18 

lenses,  9 
Squint.     See  Strabismus 
Standard  eye,  21,  25 
Staphyloma,   100,   107 
Static     lenticular     astigmatism, 

121,  123 
Stenopaic  disc,  139,  204 

slit,  115,  134,  204 

spectacles,   141 
Stevens  on  heterophoria,  174 
Strabismometer,  192 
Strabismus,   184 

alternating,    185 

angle  of,  192 

apparent,  95,  185,  187 

concomitant,   184 

constant,  185 

convergent,  91,  95,  184,  193, 
220 

divergent,   184 

heredity  in,  190 

intermittent,  186 

monolateral,  185 


Strabismus,  paralytic,  184,  192 

periodic,  186 

treatment  of,  96,   193 

vertical,   185 
Sturm,  focal  interval  of,  118 
Supplement,  fusion,  54,  102,  171 
Supra-orbital   neuralgia   in   eye- 
strain,  164 
Symmetric  astigmatism,  119 
Symptoms  of  astigmatism,  121 

of    eyestrain,  .  92,    122,    149, 
162,   173 

of  hyperopia,  92 

of  myopia,  105 

of  presbyopia,   148 

Table  of   accommodation  power 
at  different  ages,  143,  144, 

145.  147 
of  inches  and  dioptres,  14 
Tabloids,  ophthalmic,  200 
Tenon's  capsule,  39 
Tenotomy  in  heterophoria,   182 

in  strabismus,  196 
Test  for  aphakia,  159 
for  astigmatism,  125 
for  hyperopia,  95 
for  malingering,   183 
for  muscles,  44 
for  myopia,   106 
letters.  Fray's,   133,  134 
types  for  distance,  26,  202 
Dixey's,  202 
for  near  vision,  28,  203 
Testing  optical  centre  of  lenses, 

209 
Tinted  glasses,  113,  204 
Toric  lenses,  16,  160 
Trial  case,  203 
frame,  204 
Tscherning's    theory    of    accom- 
modation, 23 

Unequal    contraction    of    ciliary 
muscle,  121 
sight     in     the     eyes.      See 
Anisometropia 


Velocity  of  light,  i 
Vertical  strabismus, 
Vertigo,  167 


185 


INDEX 


243 


Virtual  focus^  11 
image,  11 

Vision,  acuity  of,  27 

in  absolute  hyperopia,  92 
in  astigmatism,  118 
in  myopia,   105 
record  of,  26 

Vision,  binocular,  42 

tests  for,  43,  153 
monocular,  43,  157 
obscurations  of,  94,  149 
testing,  apparatus  for,  202 
tests  for  the  Services,  221 

Visual  angle,  25 
axis,   187 
projection,  77 


Visual  standards  for  recruits  in 

chief  European  armies,  229 
Vitreous    opacities    in    myopia, 

105 
shrinking    of,     in    myopia, 
108 
Voigtlaender's  lens,  63 

Worth's  amblyoscope,   195 
theory  of  squint,  191 

Yellow  spot,  72 

changes  in,  in  myopia,  loS 

Zone  of  Zinn,  ^3 


Bailliire,  Tindall  &'  Cox,  8  Henrietta  Street,  Covent  Garden,  LondoH 


OPINIONS  OF  THE  PRESS 

"...  a  trustworthy  guide  for  the  student  or  the  practitioner,  and  will  enable 
him  thoroughly  to  understand  (he  principles  on  which  errors  of  refraction  and 
defects  of  the  ocular  muscles  should  be  treated  .  .  .  the  symptoms  and  diagnosis 
of  myopia  are  particularly  well  written."— Lanccf. 

"The  preface  of  this  excellent  handbook  contains  the  keynote  of  the  whole 
book  :  '  I  have  tried  to  make  the  following  pages  .  .  .  essentially  practical.' 
Whether  the  average  medical  student  can  afford  time  to  peruse  this  book  or  not,  he, 
at  all  events,  may  know  that  by  possessing  it  he  has  a  plain  practical  guide  .  .  . 
very  practical  monograph,  which  we  can  thoroughly  recommend  to  students, 
specialists,  and  general  practitioners."— M^d/caZ  Press  and  Circular. 

"  The  preliminary  chapters  dealing  with  elementary  and  physiological  optics  are 
particularly  well  written,  the  chapters  on  accommodation  and  convergence 
deserving  special  praise  ...  its  existence  is  justified  by  the  freshness  with  which 
the  subject  is  treated,  by  its  all-round  excellence,  and  by  its  conciseness  and 
lucidity." — Ediltburgh  Medical  Journal. 

"  The  book  is  singularly  free  from  clerical  blunders,  is  written  in  a  clear  and 
attractive  way,  and  is  illustrated  by  many  diagrams  and  several  plates.  We 
consider  that  Mr.  Clarke  has  produced  a  useful  and  trustworthy  book,  and  one 
that  we  can  cordially  recommend  to  students  and  to  practitioners  alike." — 
Ophthalmoscope. 

"  It  is  throughout  a  thoroughly  practical  book,  and  can  be  recommended  to 
students  requiring  a  reliable  guide  to  this  most  important  branch  of  ophthalmic 
work. " — Practitioner. 

"  It  is  simple,  concise,  and  lucid." — Dublin  Medical  Journal. 

"  It  is  clear  in  expression,  easily  readable,  and  not  much  given  to  mathematics.  ,  .  . 
Asthenopia  receives  an  adequate  share  of  attention,  such  as  it  deserves  but  does  not 
always  get"— Medical  Chronicle. 

"  It  is  thoroughly  sound  and  up  to  date.  It  is  not  too  long  and  not  too  recondite. 
The  style  exhibits  those  merits  of  simplicity  and  lucidity  which  are  essential  to 
exposition  and  explanation,  and  the  illustrations  and  diagrams  are  in  keeping  with 
the  style.  The  aspect  of  eye-work  which  we  have  endeavoured  to  indicate  we 
cordially  recommend  to  the  notice  of  our  readers  ;  and  we  can  say  with  confidence 
that  in  the  study  of  these  important  matters  it  would  be  impossible  to  find  a  guide 
more  reliable,  or  an  exponent  more  lucid  and  instructive,  than  Mr.  Ernest  Clarke 
has  shown  himself  in  these  pages  to  be." — Journal  of  Balneology. 

"General  principles  as  well  as  practical  rules  are  carefully  explained,  and  the 
recognized  difficulties  of  the  subject  receive  adequate  treatment  ...  in  all  respects 
his  book  commands  confidence,  and  we  heartily  wish  it  an  abundant  success." — 
Polyclinic  Journal. 

"The  book  is  well  written,  clear,  and  definite,  the  paper  and  printing  are  good, 
and  it  is  a  most  useful  addition  to  a  student's  library.  It  is  well  illustrated,  and 
very  lucid  in  description." — Guy's  Hospital  Gazette. 

"Those  who  are  familiar  with  the  work  of  Mr.  Ernest  Clarke  know  how  care- 
fully and  thoroughly  he  does  his  work.  This  volume  is  the  very  best  of  the  class 
that  goes  into  everything  with  care  and  thoroughness.  We  congratulate  the 
author  in  being  able  to  present  the  profession  with  such  a  complete  work  on 
refraction  in  such  comparatively  small  bulk.  The  type,  paper,  and  illustrations 
are  excellent.  The  coloured  plates  are  specially  good.  The  book  merits  the 
highest  commendation."— Canarfa  Lancet. 


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